Originally Posted by
anon2k2
I have heard (but it could just be urban legend) that the reason why airlines check your passport before boarding international segments is that the airline would be on the hook to fly the passenger back to the point of origin if the passenger couldn't produce a valid passport at the destination.
It's that, but also fact airline gets fined major amounts if passenger is flown in that isnt admissible. Note the airline only has to fly passenger back to country flight came from.
This is why some countries require same documents to transit as for entry. They're on the hook for taking the passenger in even though passenger only connected, if passenger is sent back due to being denied entry.