The particular cop in question has no doubt that there are bad apples who do bad things among the boys in blue. He knows that there is injustice committed by police officers. His point is that overwhelmingly cops don't arrest someone without a really good reason to do so. He is also--like so many people in the criminal justice system--jaded by his day to day work, primarily interacting with bad people who do bad things. He has his biases, and--unsurprisingly--generally believes in his colleagues as professionals who do the right thing. That doesn't mean he doesn't believe there are bad cops--he knows there are--but his initial instinct is to believe the cop, not the suspect.
As likely one of this board's more liberal contributors, I can say with absolute certainty that this particular cop is the kind of person I want policing my streets. He's not infallible, nor cured of any bias (we all have bias, like it or not). I take his view on this subject as a reference point, nothing more. The police need to be policed, too, and like many organizations that profess to regulate themselves, they do have blind spots with regard to their own. This is particularly problematic when trying to address societal trends that are hard to pin on any one person, but that show up very clearly in statistics for larger populations. That doesn't make them bad people, it makes them people.