How do you know Its just Apples decision and not influenced by its wireless partner networks?
Apple wanted MMS with iOS 3.0, but AT&T held them up. There's no reason Apple wouldn't want Facetime on 3G, it's the Carriers.
I gotta respectfully disagree. Apple is loved/hated for not releasing products until they live up to their standards, regardless of consumer demand (think the original iPhone being edge only because 3G chips at the time drank batteries).
It's a similar situation here. If Apple allowed everyone to use Facetime on 3G, they would be putting themselves out there to be judged as releasing a crappy product when users couldn't video chat because of poor cell reception. Facetime would be to blame by the masses and, therefore, Apple would be to blame. But instead, they allow it only over WiFi so they can (almost) always be assured of a fast internet connection and a good end-user experience. That way, all they get is "Man, I wish I could use Facetime over 3G! I don't understand why Apple won't let me!"...instead of "This Apple video chat is a joke. It never works!" This approach has been a large part of Apple's business model since Jobs took the reigns.
Then on the flipside, if it was the carrier that was forcing Apple to limit Facetime to WiFi...why on earth would they allow Netflix, Hulu, Crackle, etc, etc? They could just as easily tell Apple to not allow 3G streaming apps...but they don't.