You cannot expect people to authenticate to view them, as the best you could hope for is some wonky one-time password sent along with the message there was an MMS they could look at, and which users would not stand for.
This is exactly what ATT does with MMS messages I've received. I guess it could be seen as annoying, but it seems better than the alternatives. You could even argue that they want it to be annoying. I would imagine that people who complain are told it's for security purposes and to make things more convenient, you just need to add this MMS plan for $X/month.
I have a theory that it's impossible to prove anything, but I can't prove it.