What justifies quarantine isn't that it's only done to a small number of people, so therefore it's OK to oppress them. It's that it is implemented by a democratic government, where people have rights regardless of their health. For example, if the Italian people don't like how their government handled the pandemic, then they can elect different people with a platform of change in the next election.
Don't agree at all. Look at what you're saying -- you're justifying quarantine not on the basis of the nature of the restrictions or how its implemented, but on the type of government that's implementing it. By your logic, the most restrictive and overreaching quarantine is fine, as long as the government is democratic. But even the most modest, reasonable quarantine is unjust, if it comes from any other type of government. While there's an argument to be made that any action taken by an undemocratic government is unjust, that doesn't mean that undemocratic nations are unable to act admirably, or that all actions taken by a democratic government are just. The justness, or rightness, or validity of a quarantine can't be defined just by the type of government that imposes it. And of course it's different from a criminal conviction, because getting infected by a disease is not a matter of guilt. But otherwise the analogy holds.
Quarantines need to be justified on their own merits. The key is the balance between the threat to public health, and individual rights. Quarantines are inherently a violation of individual rights, which means the threshold for imposing a quarantine needs to be very high. That's why quarantines, in any time and any place except the crazy upside down present, are imposed on those who are sick. People who are infected by a disease, and who either show clear symptoms, or who were exposed to someone who was known to be contagious, usually via personal contact. It's also restricted only to the most severe diseases, based on a combination of infectiousness and effects like death. Since there is no guilt, it means these people must be treated with a high degree of consideration, because they're being imposed on, not punished. And because it is such a great imposition, it must be highly curtailed, and the people must be released under clearly defined conditions, and as soon as possible.
Quarantines are a violation of basic human rights, which is why they've always been highly targeted and require strong justifications. This is a clear and present danger kind of thing, not a theoretical, amorphous statistical risk. It is simply not acceptable, and not just, for basic human rights to be thrown out because of a vague mighta coulda; or to impose those restrictions on an entire population, instead of on individuals.