I agree that the whole return ticket concept is silly for many reasons. However, it's the authorities that impose these requirements. The airlines simply enforce whatever the rules are. Granted, it's a bit annoying when agents can't understand that onward travel doesn't necessarily mean a flight, but it's usually resolved after some explaining.

The most trouble I've had was flying LAX-SIN on JL. Being Japanese, every rule was followed to a T. I had to buy an onward ticket on the spot just to satisfy them. Of course, I cancelled it the moment I walked away with my BP in hand...