I'm quite saddened by Elo doing this truthfully. Squashing addon development is no way to treat your fans, whether it contained your source or not. It was done to foster development and cross-play between mods, not to cause any harm.
At the end of the day, making an addon to a mod is still an infringement of said mod creators rights unless they have been given explicit permission to do so. No matter what you or anyone else say is going to ever change that fact unless the intellectual property rights change.