You should have separated your argument between charging to run a server and charging to use their mods on your own game as they are 2 completely different things and it isn't really clear what is being done here but ultimately, I don't see an issue charging for either.

Most good server communities across many games charge money. This game in particular requires a lot of admin work because many resources do not respawn so admins need to manually respawn a lot of things in the game world in addition to this game being really easy to grief in. Being admin in this game is more work than I have seen in any other game, I certainly wouldn't do it for free.