The issue is you're paying money to a third party for mojang's work, more precisely something which you have already paid for by purchasing the game. If you ignore the game itself and look at it from a purely technical view, what people are essentially doing is running code on their own servers (Which is allowed) but then charging people for bits of the code written by mojang (Which is not allowed).

For most private servers you see, they will have a list of "donation" amounts and a "reward" for each, however this is quite clearly a sale since you are exchanging a commodity for money. To be classed as a donation, you would need to donate money without the requirement of something in return.

I think Mojang are clearly right here, but not to the advantage of the minecraft community.