In legal terms in American (and others') immigration systems, everyone who comes to the country and isn't a citizen is an immigrant. When they leave again, they're no longer an immigrant. I think they started doing that after science fiction made the term "alien" applied to a person sound really strange. Current American anti-immigration policy has been enforced strongly against, well, almost everybody, but definitely if a criminal record is involved. Since he wasn't planning to move to the country, only visit for a while, he wouldn't have the required paperwork and could be held up unnecessarily because the program that has a big list of names doesn't include much complexity in its algorithms. So the judge just wanted to send a message that this is under control, and there is no need for more chaos than they already have. We'll see if that goes anywhere.