I tend to think that ego is at the root of most controversy. Humans like being certain in their beliefs and get, at the least, squeamish when their assumptions are challenged so overtly. Most any provocateur worth their salt understands this and can use it to inform others on the hidden aspects of the human condition, those things most would rather sweep under the rug.

Some go too far, of course, as is the case with Balthus, but again, pushing boundaries is essential in any creative endeavor worth remembering. If we never push for greater clarity, we never broaden our horizons. What matters here, of course, is that people get too used to comfort and have a hard time dealing with anything which infringes on said comfort, or even the sense of entitlement one may feel toward having said comfort.

Essentially, humans can't stand having their illusions shattered so overtly, which is why some artworks are so controversial; and which is why controversy is essential. Some people need to have their illusions shattered, lest they lead others down a bad path with their naivety.