If you signed a contract saying they can alter it, they can alter it. That's all there is to it.
And it doesn't matter, anyway, as Lucas got to release his version 5 years later.
However, the 1973 cut should still be preserved for historical reasons. The fact that the 1973 cut of American Graffiti and the 1971 cut of THX 1138 have never been released on home video in any format is even worse than Star Wars' situation.