Minecraft EULA: https://mojang.com/2014/06/lets-talk-server-monetisation-the-follow-up-qa/ Look at these: (Hard currency is real-life money)
Lol Edit: It's all legal stuff to give Mojang the ability to take action if they need to. Mojang doesn't really care and isn't actively pursing servers that don't follow it.
Ah. That explains a lot. I mean a LOT. (I'm not talking about mega, mega is fairer when it comes to this.)