It is not a firearm in the USA. it is an assembly of firearms parts. Anyone can buy it, even a convicted felon who happens to be under 18 and has been ajudicated a mental defective *and* is holding a passport from Afghanistan ......
Parts are Parts.
If it then legally is attachable to your AR up in Canada, great for you! We here in the wild west would need to fill out a form, send in $200, and wait eons to be able to do that. In the interim we better not be caught in posession of both the upper and a lower, which is the "firearm" part.