No, they said they wouldn't, it's technically quite profitable for both parties involved. Steam is a different platform, their Terms of Service doesn't allow it, Rockstar Games would also re-evaluate their opinions and it would be taken down for good. It's really unlikely to stay long in there, even if intentions are good.
And yes, they are already breaking their Terms of Service, but because it's not involving the players, it's seen "fine"...
As long as MTA doesn't have micro-transactions, Rockstar and TakeTwo aren't worried.