Airport Improvement Fees (AIF) are very common in Canada, pretty much every major airport has one. Most of the time it is included in the cost of your ticket but in YUL (Montreal) and YVR (Vancouver) it is collected seperately as you found out. I believe the reason for this is as follows. It is basically up to the airline to agree to include the fee in their ticket or not to. Most of the airports have convinced the airlines to do this but obviously YUL and YVR have not been able to or have chosen not to. I think (could be wrong) that the airlines that do collect it in the ticket price either have some say in the improvements being made to the airport or agree with the improvements. If the airports don't have the airlines collect it, they can do whatever they want with the money (airport related).
Since the Canadian Goverment no longer runs the large airports (and thus no longer funds them), this is one way for the airports to fund their various improvement/construction projects.