My understanding is that trading on non-public info isn't a crime unless both parties benefit. The behavior I'm referring to was sharing tips with the expectation of getting tips in return at a later date. As far as I know, it's not possible to prove that's illegal.
Also, it's not like actual illegal behavior is uncommon when it's hard to prove, and it's not like a lot of people aren't proud of it. Watch Jim Cramer's interview with Jon Stewart as an example and the reaction to Wolf of Wall Street as another.
In business school, a friend asked me to recommend firms where he could do some quiet insider trading by tipping off relatives who would trade on his behalf. He ended up at MF Global.
There aren't statistics on this kind of thing, but my experiences have all told me succeeding as a hedge fund requires breaking the same rules as everyone else, not unlike doping in pro sports.