Of course you're in the US. However, you are in a special part of the US that's governed my an additional set of laws called "airport laws."

Among the nicer airport laws is the ability to buy things duty free (subject, of course, to you not bring them back to the "real" US).

You aren't allowed to carry a gun, even if it's otherwise legal in that state.

There are lots of things that are different in the airport.

For an extreme version, see the movie "The Terminal." Apparently, this sort of thing has happened.

So, yes, you're in the US. But that doesn't mean that you have the exact same "rights" on the different sides of the security line. Things are most definitely different on the other side.

In this case of the present discussion, the fact that they have you "quaranteened" means that they don't need to be subjected to the security procedures.

Next time you're in an airport, try this: go through security. Then, exit security and try to come back. This time say "I already went though" and refuse to be subjected to screening procedures again. See what happens.