There's a legal aspect where if you don't defend your intellectual property you may lose it.

You also don't want to set a precedent because if you let some rando do it, why not let a company do it? Why not let Google do it?

Modding implies toying with someone's IP, and the basic premise is that you can't paywall the resulting product. There's a lot of leeway and you can ask for donations, offer private beta to your patrons etc... it can definitely be cash-flow positive but a straight up paywall is a violation of the social contract that governs the modding scene.