I'll call a little BS here.
MMS messages are billed under your messaging plan.
Yes it uses a data connection to send the pictures or video, but this data stream is not billed against your data plan. AT&T's APN settings differentiate the traffic.
That's what the MMSC, MMS proxy and MMS Port settings are for.
The only way they would get billed as data is if you altered or removed this setting.
I sense you're either making this conversation up or you had a CSR who didn't know his butt from a hole in the ground.
This seems to describe you - clearly you don't know how iMessages work. Re-read the OP's original complaint...