They can ignore it and pay damages if there is no cancellation clause. It is highly unlikely a court would enjoin them from doing some as pro consumer as opening their platform to another US provider.
It doesn't make sense to work out such a contract without a cancellation clause. They could as well just make a verbal promise without signing something.
Penalties are usually very, very high if one side doesn't fulfill its part of the contract. By high i mean high enough so that Steve still sticks to AT&T ;)
