Why did Canada stamp my passport at the Montreal airport ?

good question......

It is their job

Why did you come to Montreal since you hate the city and the french population ??

Please leave, we don't need haters.

Because they have that option at their discretion.

Why not? It is kind of what they do!

So that you know, without doubt, when to leave.

Because he did not get his end away the night before

They do that sometimes, not sure why. I have been stamped (my US passport, not me personally) when entering Canada, too (at a surface crossing by car), but only once of many times. The guy even said, "well, I'll stamp you even though it isn't necessary", whatever that is supposed to mean.

Did you leave the in-transit side?

Cause they’re idiots