You could argue that because the phone is locked to AT&T's network, part of the agreement is that you'll use AT&T's service if you take the phone abroad. Basically, by selling a phone that only works on its network, AT&T is bargaining for the right to any international usage of that particular phone, if there is any.

This is a very common contractual scenario--you get a discount for agreeing to only use one particular vendor, to the extent that you have a need for the particular kind of service they provide.