Police are subject to the same biases and logical fallacies as everyone else. It is also not in dispute that they can also be arrogant and think they know-it-all. They know your client is addicted to meth; he was loitering around a broken storefront window, he had B&E tools in his bag, and he tried to run away when the police had arrived. There's a bad thing that happened. He's a bad person. Case closed?

If they stop to think about it, they might acknowledge the case being entirely circumstantial - but deep in their cop-like soul, they are compelled - of course - and more convinced that your client did it, and are irritated that you're running this ordeal over a criminal trial in a court of law. And so their testimony on the stand stops allowing for mistakes or doubt. They are now certain no one else was nearby (despite it happening on a busy street on a Friday night). They are now *certain* your high-as-a-kite client understood enough that the mere veracity and his ranting or raving was coherently a "confession" (even though his intoxication was so severe that the on-call nurse kept him under observation within a cell). They are now *absolutely confident* that when he ran, an item from the store was dropped (even though that little detail appears nowhere in their police notes, or initial intake report).

The presumption of innocence essentially vanishes, and certain officers take the attitude that it really doesn't matter what actually happened at that time - the "truth" is that your client is a criminal, because he looks like has a very long rapport of issues.