I suspect that they can defend federal building but they are doing a lot more than that. I also suspect they will be reined in and along the way get their butts sued off for their behavior by not only the city and state but the victims too. Since, apparently, this has been done under the aegis of the AG he too will get sued.
Trump has always liked a good suit and, now, he get to reap his rewards. You know, stuff like law suits, bad press, and a further diminution of his brand.
So far they Homeland Security has been sued by the state AG, the city attorney, and some victims as well. This is just starting.