MacKenzie was one of Bulger's associates and the impression I get was that it was a voyeuristic thrill for him and the other guys to watch the "activity". The gym they used was really a private facility open only to inside guys.
MacKenzie comes off as a real scumbag who had an attack of "conscience" similar to the ones most rats get when the walls close in,so who knows how involved he may have been,or if he himself ever "indulged".
As far as other criminals go,if true,this was not a widely known thing outside the inner circle. Also Bulger had enough clout and was feared to the point where no one was going to admit that they knew about it.
Again, I'm not saying that I know this to be true,I'm just asking that outside of MacKenzies and Carr's books,and the Boston Herald article,there has been any other mentions of it.