It happens on the other side as well.
I took Via Rail from Port Huron to Toronto, and the inspectors at Sarnia asked me all sorts of questions when they found out I was a writer. Who's your publisher, how much money do you have with you, in the bank at home, credit cards, where are you staying, what are you seeing, do you have a return ticket ... on and on and on. I sat there, while they put me on hold and disappeared, wondering how I'd get back to the States if they did send me home. (And this was before 9-11).
I didn't really think they would, however, like the man in the mac telling John and Yoko they had to go back. I was ready for whatever might happen. Had my little notebook with travel itinerary, and the papers I'd made notes on while planning the trip, and whipped it all out on demand. At least they were civil. The people who seem to savor being a pain in the you-know-where always go about it with more class in Canada.
In the States, I'm sure there are lots of shrill-voiced groups demanding even more of what happened at the Maine inspection station. (While it's now apparently okay to sneak across from Mexico. They want to let those folks stay even longer. I don't know how to say "I hope I get re-elected" in Spanish, but George probably does.)
The ony post 9-11 border crossing story I have is being towed back after having car trouble in Sarnia. Never came back through the commercial lane before. The tow truck driver said it happens now and then. And he explained that his mom lives on the US side. They just waved us on through. It probably helped that both of us were locals.