It’s still illegal even if you don’t do it for commercial purposes. Think of people doing things with it that you don’t like that reflects bad on your IP–that’s why many game developers don’t allow people to create games in their universe even if the game is free because it could affect people’s views of the official stuff.
Most cases they won’t do anything about it, that’s why if you get enough attention then it can be a problem. Of course though, whatever you do on your own people won’t know about.