My players in Dark Albion or Lion & Dragon are always surprised, when they're first playing, at the fact that for the most part they can't just wander around the streets of London dressed in plate mail and heavily armed.

It seems like in most D&D games, there is this notion that you can wander around any fantasy city, with no special governmental authority or whatnot, looking like you're ready to engage in mass slaughter.

And I get that not every game needs to be entirely "medieval authentic", but I have trouble imagining most states of fantasy governments functioning effectively, where people who don't have political authority get to go around in full gear.

I'm not a scholar, but wasn't armour ridiculously expensive in medieval times? How likely would you have seen people in full plate wandering about anyway?

Fantasy has its own prerequisites which are thoroughly different than those of a historical campaign. Part of the attraction of fantasy is being able to steal elements of history without most of the "downsides".

I'm prepping a 13th century Low Countries campaign right now and it's going to be very different from my usual sword & sorcery campaigns by default.

Generally, no.  Frontier towns, maybe.  Some warlike cultures might allow people to go fairly well-armed, but not any type of civilized city.  For D&D worlds, maybe cities like Highport, Molag or Dorakaa going around without full gear might just mark you as prey.

Yes

A) it's a staple of the genre (which is not "medieval authentic"). Conan, Elak, Grey Mouser/Fafhrd all wore steel openly. So did Solomon Kane for that matter.  

B) The world is more dangerous, with monsters and such. People need to be armed, especially those that deal with monsters. In most D&D worlds, the state doesn't protect people from monsters and such, armed adventurers do.

C) People aren't serfs/slaves/conquered people (like in medieval England), they are free.  Carrying weapons is a right of free people. Hell, the state I live in lets people open and conceal carry without a permit. It's quite common to see people toting firearms on a holster in public. It might be considered gauche or unsophisticated, but again, in a D&D world, people would have far greater need to have weapons than ours.

Wearing steel or armed with a sidearm is different from walking around in full harness w/shield or carrying pole arms, greataxes, or longbow and quiver at the local watering hole.

No; in my historical game, only citizens of Massalia were allowed to go armed and armoured within the city walls. Even then, it was socially frowned upon for citizens to do so unless there was trouble.

I guess so, I don't often run adventures set in cities though. Exception would be Paizo Golarion stuff, I remember a Bard who walked around with a halberd!

Back when I ran Lankhmar in the '80s the PCs would wear leather & carry side arms like maces & swords, though. But players will buy and equip anything in the PHB. If you don't want them wearing plate, don't put it in the equipment list.

Almost always no.  I remember reading the Referee advice in Cyberpunk 2.0.2.0. about punishing players walking the streets in MetalGear armour with a heavy assault rifle shoulder slung and the mentality has stuck.  Keep your armour vest in the car boot, your combat gear in a backpack you can ditch and your dagger in your boot.

I have no problem making characters rot in a cell, pay heavy fines or be preemptively declared troublemakers and denied entry to where the important things are happening.

Player characters walking around in full armour, upsetting the proper order? FEAR to the rescue!



(From City State of the Invincible Overlord, 1976)

Another origin of the practice is "killer" DMs seeing unarmored/unarmed PCs as an opportunity to "get 'em". Going about a city unarmored is an invitation for such DMs to take advantage of the PCs' vulnerability - not to create adventure, but to abuse the PCs. Hence, "keeping their defenses up". It's an artifact of poor gameplay.

In my campaigns, it varies wildly depending upon the nature of the campaign.  For the current one, I turned the idea on its head, as a natural consequence of the setting:  People are relatively scarce and valuable, because portals can open up into the fey and shadow lands almost anywhere, with rapid and nasty consequences at times, never mind other magical and fantastical nastiness.  "Adventurers" role in society is to deal with such things.  They are essentially a semi-official, but mostly autonomous social class charged with dealing with special trouble as it arises.  An adventurer might not (and probably will not) have full combat gear in a town, the same way they wouldn't have it in most dungeons.  They will be armed and armored.  One that doesn't is considered to be a coward, derelict in their social obligations, or both.  An adventuring group is the first thing deputized when a local ruler needs backup.

In other campaigns, I've had stringent rules, and in most campaigns, it is somewhere in the middle.  Very much depends upon the setting.

One of my citys that deals with a lot of adventures has the rules posted.  

No shields
No heavy armor
No spells above 3rd level with out a license
No evocation
No deamon summoning
....

A buch of other stuff.

Mostly I forget/am less concerned about sidearms in cities (and therefore all the town guards in my head are also less concerned). Armour though, gets a verbal warning followed by a de-armouring, and knowing, never mind using, damaging cantrips in my 5e game is a capital offence in most civilised cultures.

Depends on the location.  If it's a city with a strong centralized government, often the only arms allowed are swords (long, broad, and short), clubs, and daggers.

Which just happen to be the weapons allowed to thieves in the 1E PHB.

Depends on the setting and location within the setting.

Generally, the main settings for our fantasy games generally run more like the American Wild West sans firearms + spells than something overtly feudal. Freemen are the norm, not the exception and the definition of a Lord is any wealthy landowner who rents out their land to tenants (i.e. a literal Land Lord). Government is local and decentralized and you'd best have the arms and armor to defend what's yours because the Sheriff might be 20+ minutes away when bandits or a wild beast come looking to take what's yours.

Throw in the fact that magic and shape-shifting beasts are pretty pervasive in the setting and "looks unarmed and unarmored" could as easily be "massively powerful wizard who could level the entire town" as it could "harmless old lady" and letting the populace have something to at least try to stick into an evil wizard/demon when they start chanting and waving their arms around is a prudent gesture.

This is a Magic Free Zone laws work about as well as Gun Free Zones do in the real world; they only keep out the law abiding and none of the realms are usually strong enough to survive that level of idiocy for long without getting overthrown.

In our most recent setting the concept of the hereditary monarchy (vs. the warrior class electing the king's successor from among themselves) is only starting to take hold in some of the more civilized areas in the last generation or so and they still risk falling prey to some warlord with ambitions if they aren't up to the task.