I don't think they are too worried about it. They are not working, and are spending money.
I have a friend who lives just over the border near Vancouver, the road he enters the US on each week to pick up the mail that arrives at a box he keeps on this side of the border has no INS/Customs.
My husband and I got a bit scrutinized on re-entry a few years ago, they searched our car but there weren't any armed guards pointing automatic weapons at us. I think the five foot inflatable Bruce the Shark which had been autographed by about thirty people maybe gave them a bit of concern. In the end they didn't even charge us duty on the three cases of wine and three cases of beer we had declared when we first got to the border. Of course they didn't find the other contraband of the um, smokable type (legal in Canada, not in the US, and not drugs) that we were bringing back for a friend. That was the only time I ever got more than a cursory glance at my passport.
I've crossed the border six times r/t in the past 4 1/2 years, at YYC, YYZ, WPG, and by car at Lewiston three times with no problem. I travel on a US passport, and I know I'm in the Canadian system, as I've held a temporary Visa and working papers in the past, and have a Canadian Social Insurance card. Like I said, they usually give half a glance at our passporst and wave us on. It's really not that big a deal.
Anne