User blog comment:Avalair/Bye-bye Customs/@comment-24772962-20150401220131/@comment-25995065-20150401220650

It's not that it necessarily breaks the TOU itself (I think), but some of the ways it's been used, such as customizing parts of user pages/message walls and whatnot, does break the TOU. Therefore Wikia will just remove it completely rather than compromise with the community to fix the problem itself instead of breaking half the site's templates and styles.