Exactly. Apple has more influence over the cellular industry in America than any single company including the carriers themselves.

They could completely design the spec by themselves with all the pro-consumer features that one could imagine including things like distributed spam filtering and then force the carriers to adopt it either by using the stick (force adoption by X date or lose the iPhone) or the carrot (shift proprietary iMessage features to the new standard so that users demand the carrier adopt it -- basically green bubble any carrier that doesn't use the standard). No other company in the country could do this.

But of course, complete control over a proprietary iMessage protocol that they can lock competitors out of is apart of the business strategy.