Basically. I work for the CIA's Joint Terrorism Task Force.
There is absolutely nothing unlawful about detaining a suspect for up to 48 hours. We have cause, even if it's slightly circumstantial. It's no better or worse than the grounds Zemo was detained on, and Mike doesn't seem to be complaining too loudly, for whatever consequences they received.
Maximoff'll be out in the morning if there are more murders, done and done.