[goodcanadian]

The airline checks passports on the way out as they are required to bring you back if you are denied entry at the destination. They try hard not to pay that cost, so they want to make sure you have a passport, all required visas, haven't been barred from the destination, and so on.

[wila]

Normally they do not really pay tor that cost either as your return ticket is void after being denied entry. But it is indeed inconvenient for them.

[goodcanadian]

Fair point, but it does presuppose you have a return ticket, which isn't always the case.