“The question is, why were they so convinced?”
Were they ‘convinced’?
My money is on ‘they are sadistic bastards’.
Occam’s razor.
Maybe it is just that they are sadistic, but then why did they pick on this particular citizen? Did he sass them? If so, after the exam that was done under a questionable warrant, they could send him home and heckle him on the way out. Doing a colonoscopy on a citizen who seems clean because of sadism is a career ending form of the sociopathology. You only get to do it once. And there were two cops involved, not just one crazy, out of control cop. I don't think sadism explains it, although it might explain how they live with themselves after doing this to people without cause.
Did they get a bad tip? Something made them think this guy really was holding, I think, and they were so convinced, they were willing to go to any lengths to prove it. Now they'll be unemployed and the county's taxpayers will be out millions.