What am I missing here?

What am I missing here? Why can't ICE position themselves around the places where these bad guys are and pick them up when they walk out the door. of the detainment facilities. I understand sanctuary cities ignore detainer request but they can't stop ICE from making a collar as they exit the place. How about just parking an ICE agent in the building and cuff the bad guys when they are still in the place but no longer in custody. For sure the targets are unarmed at this point. I suspect there is a flaw in my thinking or it would have already been done. I just not sure what I am missing.