There's lots of MP mods out there that you'll probably never hear of. They're created by people wanting to learn how to program, and they think that creating an MP mod for SA would be a fun way to it. Noone has the right to discourage this.
So far there hasn't been much copying with SA, unless you're generalising with creating a multiplayer for GTA. But this idea in itself is not the sole right, or idea, of MTA. No group has announced intention to create a race mode, placement editor and SDK.
Lastly, I don't think MTA is in a position to call other MP mods immitations, when it failed to deliver on its promises of releasing MTA:Blue for Vice City.
http://www.mtablue.info/ <- Vaporware. Sorry to have to put it so bluntly, but it's an undeniable truth that MTA must wear. If MTA:Blue for VC had been released, there may not have been GTA:C or VC-MP mods.