Imagine if Ford or GM made lawsuits for painting your car. That would be ridiculous. Additionally, imagine if the EULA for a car was the same as for a video game. You could not even change out your air filter without going to an authorized dealer, and there would be no third party auto parts. Companies like Autozone, Advanced, and NAPA would be law breakers, not to mention all of the after market car parts you can purchase. That is an example from one industry among many where they benefit from third party "modders." Software companies need to get a clue.
Some companies are getting a clue. There are a multitude of programmers out there making 3rd party addons to Microsoft and Autodesk software and selling them legally. I don't see how making an addon for a game (selling it or not) should be any different.. .
But there is a difference, subtle as it may be. Games are the point at which programmers and artists collide. Making an addon for a game is implying the "artist" did not do as good a job as the user would have liked. That hurts feelings. There was a thread a while back about how developers were upset because they had spent alot of time and money to develop ship models and people were going in and changing, upgrading, or replacing them.
My response is that they spent too much time and money for too low of quality models. If an artist can't stomach the real world outside of their personal galleries where people around the world are able to express their true opinions about their work, they need to go back to their little support groups and liberal huddle of a pre-school where everyone has to be nice and accepting of their finger-paints and modern "art."