There WAS a ban on non-US reg non-air carrier operations. something about people flying in with A/C full of who knows what going who knows where (talk about open skies!). The ban on part 91 operations by foreign reg A/C is not total, you have to file a flight plan and get a clearance, kind of like a prior permission authorization for a military airfield used to be.
If other countries want to let N reg aircraft operate, that is their call. Since so many do I guess they just recognize the value/quality/capability/whatever that comes with it.
If Canada is to be considered like a 51st state why don't they harmonize their air regulation and commerce with US, make it one big market rather than one big (US) and one dinky (Canada) and provide the benefits of competition to our neighbors to the north. Let's see, those benefits are lower fares and more choices if you are flying from one big hub to another. If you are flying from East Jeasus to Podunk forget it. Hmmm lots of East Jeasuss and Podunks in Canada so maybe that wouldn't be so hot. But then NAVCANADA could run FAA ATC as a partial privatized organization