Quarantine is not imprisonment, and is certainly not illegal. In fact, it is customary if you, for example, sail a boat from one country to another.
The point here is that it's not clear that there's a reason to enforce a quarantine- yet. But if it becomes clear, then we need to have the balls to do it.
Are they free to leave whenever they wish? No? Sure sounds like imprisonment. Maybe it's for the best, but that doesn't mean it's not imprisonment. It's on the same moral spectrum as conscription and forced labour; the removal of fundamental rights from an individual for the betterment of a great many other people. I'm not saying it shouldn't be done, but it should be recognised that under the social system the country in question (USA) subscribes to, it's a fundamentally evil act.
Detaining people against their will is the definition of imprisonment.
While not illegal there should be comprehensive legal safeguards around it. And, if it's not illegal, what are the laws that regulate quarantine in the US?
The sub pages are lengthy and cite specific laws and so on.
Skimming, CDC has broad authority to detain travelers and states and local health departments usually have the power to enforce a quarantine (with violation of the quarantine being a misdemeanor).
The point here is that it's not clear that there's a reason to enforce a quarantine- yet. But if it becomes clear, then we need to have the balls to do it.