I don't understand why so many people have trouble getting this, so I'm going to explain it in detail.
There are TWO basic services that your phone uses over cellular:
- Voice
- Data.
When you call someone, you are obviously using voice. However, what most people don't realize is that when you text someone, from the network's perspective you are sending a different kind of "call", delivering the text message to their phone. This is why text messages are limited to 160 characters; they are transmitted through the call setup/signalling channel which is extremely bandwidth-limited.
Data is entirely different. In order to transmit information that is not an SMS or a voice call, your phone has to use the data network. There have been several data networks in the history of cellular communications (GPRS, EDGE, etc.) but they all do the same thing.
Data over cellular is transmitted through a router called an APN, or "Access Point Name". There can be different APN's for different types of data service. For example, a provider might have:
- WAP APN, used for browsing the web from a cellular phone, with proxies to reduce bandwidth use. Most providers don't use this anymore since smart phones can handle the "full internet" so to speak.
- ISP APN, used for general Internet access, such as a smart phone, or when you tether.
- MMS APN, used for transmitting MMS messages.
See that last one? MMS messages still use your data connection, but they go through a different APN. This is why your provider doesn't deduct them from your data plan. But you STILL need to have cellular data turned on because it's.. well, data (image data, and MMS texts longer than 160 characters) and you can't send that stuff over the signaling channel above.
And you can't send MMS over WiFi because the MMS APN is simply *not reachable* from anywhere other than the provider's cellular network.
It really is that simple. Apple isn't trying to pull one over on you, nor is your provider. The ONLY thing that can be physically sent without cellular data turned on are SMS messages and voice calls. PERIOD.
I hope this makes some sense of this for folks who are still confused.