While there are some 'free' (subsidized over two years) android phones available, most of those are low-performance devices more akin to feature phones. Those purchasers may indeed be less likely to buy apps due to spending habits. But there are plenty of Android phones above $200, even on contract. And a 3GS can now be had for $49.

I think it's also that Android users tend to use their phones more as solely communications devices (FB, Twitter, news, email) and less as an 'app console' device, for lack of a better description.