They have been, many times.
There is, in fact, a separate detention unit in many of these detention centres, as you will have heard about, in which they detain U.S. citizens. They know they're detaining U.S. citizens, and they put them in a separate part of the facility pending their ultimate release. It's no secret that U.S. citizens have also been apprehended—kidnapped, if you will—and arbitrarily and unlawfully detained.