Because the detention facilities don't house exclusively illegal immigrants (in fact, given the fact that US citizens have been detained in them for extended periods despite clear proof of citizenship, even calling them immigrant detention facilities is somewhat misleading.)
A more accurate label would be “arbitrary detention facilities”.
> given the fact that US citizens have been detained in them for extended periods despite clear proof of citizenship
I'd be curious to hear more about these cases as well as their proportion relative to the illegal immigrants detained at these facilities. I'm not aware of lawful residents being detained in such proportions that this would suddenly warrant tipping the terminology on the side of indetermination.
A more accurate label would be “arbitrary detention facilities”.