It's really a two edged sword isn't it?
While they may be American citizens - they're also suspected of being aligned with a group that have vowed to destroy America. That's called TREASON... I think it's perfectly alright to require some questioning to return..
You know, there was this foregin student who was living in the hotel across the street from the WTC. He was evacuated with everybody else on 9/11. A security guard found a high-tech radio/walkie talkie in his room.
The FBI gave him a polygraph test, denied him access to a lawyer, under intense interrogation he admitted to owning the radio - changing his story several different times about how he came into possession of the radio.
The FBI charged him with lying, threw him in solitary confinement for a month. He was released when the security guard said he made the whole thing up.