One of the main problems with monetizing content on the Google Play Store, whether it's apps, books, music or videos, is that many users don't have a Google Wallet (former Checkout) account. It's much easy to buy something with one or two clicks when you already have a payment method set-up.
Since Google isn't allowing something more popular like Paypal, users usually need a Google Wallet account to pay for stuff in the Google Pay Store. The problem is they can't use Google Wallet for much else right now, so many of them don't bother setting one up, and later they'd rather go for the free versions of apps, books, and so on than set up their account on the spot.
Google needs to make the Google Wallet account more popular, which I believe they will when more NFC phones arrive, and also more phones with ICS, or more of them get upgraded to ICS. That's because in ICS they ask you in the beginning if you want to set-up a Google Wallet account, and it's the best time to do it if you ever intend to buy anything at all from the Play Store: apps, songs, books or movies.
Until then, they are now announcing that you will be able to buy books, music and videos through carrier billing in certain countries, which could be easier for some people to just add the cost of the purchases on their monthly bill. In US it seems only Verizon has refused to do it. It's strange, but lately I haven't seen Verizon be the best ally of Google, and they've also been refusing to allow Google Wallet on their phones, which is just strong and DoJ should probably look into that.