I suspect they had to, initially, and the only terms they could get were for several years (basically until now). Every US phone is aligned with a carrier. It

I suspect they had to, initially, and the only terms they could get were for several years (basically until now). Every US phone is aligned with a carrier. It's not immediately clear to me why that is, but there you go. When the contract is up, Apple has a serious opportunity to shake that notion, but I'm not sure if they will.