it may not be required for *Americans* to have DL & BC or PP to re enter America... but it IS required for Canadians to have same. and like wise for Americans to enter Canada. the only thing is sometimes they don't call you on those requirements. ever since 9/11 I have had to show the documents to enter US. AND usually but not always coming back into Canada.
usually if you are Canadioan coming back into Canada though, they are more concerned with how much you bought so they know how much duty to charge you.
now that the CDN dollar is at a 10 year high vs the American $ they are not as impressed by your tourist dollars spending power too so they are more picky about who they turn back.
also the border guards are all cop rejects anyway. they see the US border guards getting guns and so on and they get jealous and try and be bad ass to make up for it.
"To make a border guard they take an incomptent policeman, drill out the skull, and pack the cavity with horseshit" - more or less Geordie Smaill quote c. 1975.