Having first-class OS support for the payment method (and the fact that a huge amount of people have been forced to register a credit card through iTunes at one point or another) means that the consumer barrier to entry is super low.
I'll never forget the day they held a gun to my head and made me buy my first iPod.
Just so I get the terminology right though, when you register a card within itunes Apple is forcing you to do so, but when you register it with Google Play, Google is reluctantly allowing you to do it. Did I get that right?
In some countries there have been periods where the only way to make an iTunes account (and thus get access to app store) was through registering a credit card or buying an itunes gift card.
Google Play does not ask for a credit card unless you actually make a purchase.
