Just out of curiosity, does it work in the web interface? AKA, if you buy a movie, can you open up youtube.com and watch the movie in your web browser?
I don't have any kind of proof, so take this with a grain of salt, but the only answer that makes sense to me is that Google made an exclusivity deal with Verizon. Granted a badly publisized one, but why else would only one version of one tablet have this feature. As a wifi xoom owner I find it frustrating, as I'm sure you do too.