Earlier quoted context omitted.
> Why doesn't this happen on iMessage? Because iMessage is not MMS - it uses a custom protocol and works completely independently from the carriers.
Fair enough, I admittedly haven't looked the iMessage protocol so I don't know much about it. I just know that if you have an iPhone, you have to set your MMSC to Apple's servers versus the carrier's servers.
I don't believe this is true. To the best of my knowledge (and a quick Google) Apple has no MMSC servers. I've certainly never input any.
SMS and MMS sent on iOS use the same servers an Android phone would. But when the target phone number is detected as being iMessage capable that whole system is ignored and iMessage is used instead.