Quebec registration plates prohibited in front? 

But... why? What bad would it do to put another plate in front? I don't understand.   
That is a question to ask the Québec government for their specific reasoning on that. I know for a fact safety is one reason some jurisdictions give. Example: A front plate might look "real" and be confused for a real plate in other jurisdictions that have a front plate, you can take your pick - i'm just hypothesizing why. I do know that some vehicles in Québec, Semis (tractor trailers) are required to have a front plate, as the trailer hides the plate on the back of a Truck (tractor), for example. Like I said, just because it's not legal, it does not mean police enforce it, and just because it hasn't been enforced by some police does not mean it will not be by others.