Immigration requirement, not a requirement of the airline.

It weeds out those backpackers that can only afford a 1 way ticket and thus cannot return home, or those without intention of leaving, thus at a higher risk of an overstay and being a burden to the state. Another similarly aligned rule some countries have is to show sufficient funds. Same kind of thing.

You would be surprised with the number of travellers who are not travelling for business or pleasure.... with an ulterior motive.

Of course it's easy to cancel a ticket afterwards, but again, rule exists for those who cannot afford to leave, not for those that can.