They're terrified of losing their blank check to behave this way. The more that people suspect that police are extremely dangerous and stop giving them the benefit of the doubt, the more chance we have of changing how they operate.
When I moved to the US, a friend's dad, a high-powered attorney (and former prosecutor) in our region, told a group of us: "don't ever talk to a police officer unless you absolutely have to. They can make your life hell for any reason they want, or no reason, and they'll always get away with it."
Coming from a country where the cops were much, much less violent and much more trusted, this was shocking to me. But that was a long time ago now and I can't honestly say the advice has served me poorly.