There is always additional charges for MMS, even if you HAVE a data plan. Gets taken out of your monthly credit unlike SMSs (which may be unlimited depending on your plan)
(Again, this is Australia, maybe different in America)
In the case of pricing, way different. I'm thinking back to the "old days" of MMS + SMS, but there was no separate charge for MMS. And actually, re-reading your post, it is the same today - unlimited messages includes both SMS and MMS. From what I have gathered over the years, Americans have both SMS and MMS lumped together - and for the longest time, I don't think regular consumers even knew what SMS/MMS stood for (that may still be true here). In Europe at the very least, though I imagine everywhere else in the world, I've come to understand it has ALWAYS been known as SMS and MMS. Not simple "messaging". Either way, my point is still that both "messages" and "picture messages" have always been lumped together, provided my memory is serving me correctly.
Metalchick, we will have to agree to disagree, as we have both been with T-Mobile for practically the same amount of time (I think you beat me by a few months). I despised data plans for the longest time, generally maintaining that it was "stupid" to try to use the internet on a phone when it was much faster and convenient on a computer. Yet I still cherished and received MMS without any additional charges. The only reason I ever even got a data plan is because it was required with my Windows Mobile Touch Pro 2 in 2009. Stupid requirements.
To express my opinion in relation to the overall thread topic... I am personally confident that the OP has a knockoff phone and was ripped off. Completely. That may or may not have anything to do with MMS. And the OP may even have a copy of WP8 installed to the knockoff phone via a ROM or something - assuming it's even actually WP8 (c'mon... anything can be faked these days). I want pics of the phone, more than just the about screen.