[bdamm]

As a Canadian that is also a US permanent resident, yes, ICE and CBP have broad authority to set up checkpoints pretty much wherever they please. "100 miles" includes the coastline. They also can do warrant-free detention, and there have been cases of US citizens detained, sometimes for over a week IIRC, over nothing more than suspicion on the part of the ICE officer than the person in question wasn't actually a citizen.

Where it gets dicey is that they can stop you, but constitutionally they can't force you to produce documentation or detain you. But not many people actually know that, or can stomach saying no to federal officers, and so in practice they can.

It is a part of US law that has a great deal of grey area. The policy was retained under Obama and Trump expanded it although not really by very much. In general the abuses seem to be limited but that's not a great deal of consolation.