Why not? As far as I know the server doesn't contain any third party components that would also have to be opened/released, and the client can keep being closed source. Mojang, as the creator of the game, can do what they want with their code, so it doesn't matter if the client contains parts of the server code. Just dual-license it, with a do-whatever-you-want license for themselves only. But in the end, I don't think it's important if it's GPL or not. Just that it is FOSS.