But then TBH why would WOTC really protest if they publish Mercer worldbooks and it's probably biggest advertisement of D&D ever
Because, according to US IP law, if you don't defend trademarks, you lose them. So the moment WotC turns a blind eye to one group using beholders, everyone can use them.
Also because the show really isn’t so much a great advertisement for 5e as it for general big damn heroes fantasy roleplaying. Because the show isn’t trying to be meta about the story actually being a game session, but instead treating it like a traditional fantasy narrative (including changing events, removing/changing the many deaths/raise deads the party suffered, adding foreshadowing that wasn’t originally there and, most notably, portraying what were level 13+ PCs as a new groups of starting adventurers in the animated series).
Anyone starting a new PC in 5e and expecting to have adventures that look anything like what was shown on the screen with Vox Machina would be bitterly disappointed.
The Vox Machina setting also being basically 4E’s cosmology with the names filed off (a result of the one-shot that started the thing off being done in 4E before switching to PF for the ongoing pre-streamed campaign… yeah, the PCs were converted twice before they appeared on the CR podcasts… that’s definitely gonna muddy the more direct comparisons… they had to write up a custom class just to make Percy work in 5e like you’d see from OSR GM’s) also runs pretty counter to the Forgotten Realms/The Great Wheel also makes it feel less like D&D (in the same way that 4E is said to not feel like D&D).
Basically, it’s only an advertisement for 5e in the sense that 5e is default fantasy rpg on the market and in the long run I think there’s a market for those who like roleplaying who come into 5e, but dislike that they can’t really pull of Vox Machina Animated in 5e without a lot of houseruling.