Well, there is also still copyright, and that allows them do remake their stuff like this, and they don't have the power to stop other studios from doing the same with their work, as that would indeed be monopoly abuse AND copyright violation. The other studios simply chose not to do this. Disney is now indeed absorbing more and more and more and more.... However they don't have an official monopoly yet, but I *am* worried...
I do know the U.S. laws have very strict laws to prevent monopolies...I've already seen some charges issued against Microsoft regarding that law....