I thought it was pretty straightforward. He used his entourage to violate the Mann Act. In doing so, he involved way too many people in his fukkery, and turned it from general sleaziness into a fukking criminal enterprise.
Also, note this part, subsection c under "Methods and Means of the Enterprise": "obtaining sensitive information about members and associates of the Enterprise to maintain control over them". I read this as Kelly blackmailing his goons to go along with his activities, which caused at least one to flip.