Not entirely true. They do offer service contracts, it's just bundled/pitched a different way. You can in fact get a phone up-front without a high cost with a contract. They have to offer that, or they'd get no new customers since the US market is used to having subsidized phone.

The only real difference is that you can get an unsubsidized plan that's not pre-paid. (Though the pre-paid plans still tend to be a better deal, with more options, if you're not in a shared/family plan.)

FWIW: In a few many major cities now, T-Mobile is updating their towers and providing some 3G service in the 1900 bands used by AT&T in the past. N9 users in those markets could switch to T-Mobile and still keep 3G. A friend of mine just got an unlocked P7000 and it's getting 3G on T-Mobil service, despite it being an AT&T branded/banded phone.