"Why do they allow women about to give birth to get on board airplanes..."
Sometimes the birth comes ahead of the expected time and it is not obvious at boarding that a person is about to give birth. Most, if not all carriers do have policies regarding how late a pregnant woman may travel and whether a doctor's certificate is required. But as you say, this is not the first time that a birth has occurred on an aircraft. Emirates has had to divert an aircraft in the past when another Philippina gave birth inflight. Apparently the cost of medical treatment is much lower in the Philippines than it is in the UAE.
As to the free travel for life: it is a myth. An airline may choose to offer it and in a similar incident in 2009, AirAsia offered free flights for life to both the baby and the mother. But there is no law mandating such an offer.
The International Community isn't even in agreement on the nationality of a child born in flight (very few countries have signed up to the 1961 Convention on the Reduction of Statelessness) so it would hardly impose such a cost on individual airlines.
“Not to laugh, not to cry, not to hate, but understand.” Spinoza