As you say, there are loads of places that sell them here; also Pyramyd Air and Airgun Depot ship them from the US to Ca. They have lists of "pre-cleared" items that can cross the border without any problems.
The key thing is they are below 500 fps. A specific muzzle energy is also involved and I think they have to be over 366 fps too.
When these things are all met, they become non-licensed airguns rather than replicas. In other words, if they were spitting out a low velocity projectile or even no projectile at all, they would be illegal replicas. Because they shoot something up to 500 fps, they are airguns even if they are... er... "replicas".
That's my understanding anyway. Clear as mud and don't quote me in case it's all wrong...