One thing that surprised me when I went to the US consulate in Montreal to get a new passport was that there were maybe 2 or 3 people in the passport line, there were maybe 30 or 40 in the room waiting to be interviewed for a visa. Since Canadians don’t need visas, I assume they were all immigrants to Canada.
Incidentally, Canada just instituted a visa requirement for Mexicans since so many had started to come to Canada and disappear. I guess it was easier than getting to the US.
Four groups: Visa Waiver Program, Visa required, Canadians, and Terry Gilliam. 
Gilliam renounced his US citizenship and claims the US government retaliated by allowing him to stay in the US only 30 days in any year, unlike the 90 days allowed for other Britons. He renounced his citizenship partly out of protest against the crazy practice of holding US citizens forever liable for US taxes even if they become citizens and permanent residents of another country.
Sounds like he has…Munchausen Syndrome!
It’s also possible that the Mexicans were flying to Canada and then crossing the northern border to the United States.
I’ve read about Gillian’s claims and two things strike me - one is that his situation is always compared to “any other European” rather than anyone else who has renounced their citizenship under the same law that was in effect when he renounced his. The second is what “can’t” actually means - my understanding is that he could have spent as long as he wanted in the US but more than 30 days would affect his tax situation (which would make sense)
Since they wouldn’t have Canadian passports, it’s not clear that getting in through the northern border would be any easier than the southern one, unless they were planning on crossing illegally. I’m not sure how feasible that is these days.
Yes, I meant that they would cross illegally. I’d imagine it’s easier to cross the northern border surreptitiously than the southern one.