True... I don't entirely buy it myself, but what other reason would Apple have to restrict Facetime other than carriers saying they don't want it enabled on their networks?
Lack of development resource.
We know that most of the iOS apps don't have dedicated development teams, and hence when work is considered "done" the team is dissolved and moved to other projects - that's why the Remote app didn't get retina support for six months. It only had one developer and they were doing something else when iOS4 shipped.
Effectively Facetime was left unfinished and the staff obviously went off to do other things - which is why the fundamentals of sorting out presence on a specific device have never really been fixed, the promised "open standard" turned out to be a total lie, and 3G support has never been added.
Phazer