Don't know how it can hurt their revenue, they don't have a competing product. They don't offer video calling services.
If you're saying that people will use FaceTime rather than making a call, the amount will be tiny. I'd say that FaceTime is one of the least used features in iOS for most people. There are a niche of users that use it, but the vast majority don't, so it doesn't make any difference to AT&T from that standpoint.
They want people off the unlimited plans, and rather than just force people off, they give the incentive to move. The customers where FaceTime is important to them, will be users of more bandwidth, therefore it's more an importance to get them onto limited plans. The people (like me) who don't really care if I can make a FaceTime call over cellular would be less likely to use as much bandwidth, so they don't care as much if "we" stay on unlimited.
It's all about managing the network. If you use a lot of data, they want you on specific plans.
It's a little ironic I think that people were completely fine with only using it over Wifi when Apple said so, but now up in arms that AT&T forces people to a shared plan to use it.