I think, as patio11 states above, it's because it's very hard to do without losing money to fraud hand over fist. It's also why Apple/iTunes is so important an

I think, as patio11 states above, it's because it's very hard to do without losing money to fraud hand over fist. It's also why Apple/iTunes is so important and why the mobile carriers should be rubbing their hands with glee (if they had any sense).