"The EULA for Minecraft says you can’t make money of Minecraft. If you make mods, they have to be free." I don't understand the legal theory behind this. Is this contract law? If a programmer wrote a third-party mod without buying the software (say by reading an API description), would they still be bound to its terms? Also, does restricting modification like this infringe on "first sale" rights?
The causes of action are: direct and secondary infringement of registered copyrights, trafficking in circumvention devices, breach of contract, and intentional interference with contractual relations.
A couple of those might not apply in a clean room implementation, but the others would.