That was a post-9/11 thing, and came from the US side as I recall. Used to be able to enter with just a driver's license. Then you could only do that at the land border. Then it was passport only.
And at each point the volume of hostile tone from US homeland security guards has increased. It can be quite stressful crossing at times and I generally avoid it, even though I'm only 45 minutes from the border.
Our family in northern maine used to go shopping in Canada every few weekends to abuse the weak Canadian Dollar, but then those damn terrorists came in through the exact border crossing we would use and wouldn't you know it, crossings got a little less liberal after that.
