Or because they've been cheating at least twice. Then there are things like Rodney Harrison getting caught using HGH, but only because he was dumb enough to use his own name and address--on an incarnation of that team where old vets like he, Antowain Smith, Seau, etc. seemed to go and miraculously regain a step.
Personally, I think it's a bit of "all the above"--they cheat wherever and however they can. You have an owner, coach and star who are sociopathic about it.