Mojang sets guidelines for how servers can raise money in order to pay for their operating costs. They say server administrators are allowed to charge for access to servers, accept donations, and have advertisements on their servers, but are not allowed to give users who donate unfair advantages:
These rules are making attempts to prevent Minecraft servers becoming “pay-to-win.” We hate the idea of server hosts restricting Minecraft’s features to players who have already bought our game! It seems really mean.
Notch clarifies that this includes servers with custom gameplay modes that are not present in vanilla Minecraft:
@PandaToothGamin You can't made paid games based on our engine. That's the entire point.
— Markus Persson (@notch) June 7, 2014