Maybe they want to avoid players who do things like...
1. being involved in a double-murder, where the defense was that the player was surrounded by the wrong people. (Ray Lewis)
2. being involved in a dog-fighting and gambling scandal, where the defense was that the player was surrounded by the wrong people. (Michael Vick)
3. carrying illegal weapons into nightclubs and shooting it(Plaxico Burress)
4. surrounded by underage drinkers and being alleged to have raped them (Big Ben)
5. being involved in a drug ring (Jamaal Lewis)
6. drunk driving...(numerous)
need I go on?