And why are heavy fines imposed on the airline? To ensure that they do not allow passengers to fly here without the onward ticket. Imagine the chaos that would ensue if passengers turned up at Manila airport without the ticket. There are no facilities for arranging an onward ticket airside at Manila airport. It would be an impossible situation. So to avoid that, airlines are under strict instructions, with heavy fines etc, if they allow this to happen. Hence they ensure that no passengers is allowed to board without the ticket.
David.
it is not just the Philippines (.....
they do not allow passengers to fly here without the onward ticket. ..... ) that require on ongoing ticket. There are many countries which have the same requirement before issuing a visa, whether prior or on-demand at the entry point.
It is not because the Philippines can't handle the influx (which they probably can't anyway

), but rather a universal Immigration issue to stop neer-do-wells, or those with no visible means of support, from entering any country and abusing whatever social welfare system exists.
With an ongoing ticket requirement, the nation's Immigration Department are assured the tourist/visitor can depart - if they run out of money, for instance.
Most, if not all, Asean countries require non-Asean citizens/visitors/tourists to have ongoing tickets. Other countries also have similar non-citizen, or non-community (eg. EU/GCC) member, requlations.
Peter