The problem is that it is hard (impossible) to tell from within an iOS app if it has been purchased legitimately. If you want to give users the convenience to buy the app for $.99 and then start playing, then I don't think you have a chance to add any meaningful validation. I wonder how it would've worked with subscription IAPs. I would guess that they are a big psychological problem to potential legit players. This would have been a great chance to find out.

In my experience, piracy on iOS varies a lot with the region, too. The majority of my analytics come from China with only one(!) copy ever being sold there.