But that's the thing, we rely on the police to do these things, and in order for that to happen we need to trust them.
Let's say for example you called the police because someone is trying to kick your door down. You are vulnerable, upset, say something not illegal but could be taken as an insult if you squint very hard and they arrest you, and physically hurt you.
Is that okay because they came out when you called?
Or the woman who thinks she was raped in police custody. If that turns out to be correct is it okay that she was raped because the police came out when they were called?
So we have a police force but if they wrongly arrest you on a whim, hurt you or rape you you aren't allowed to complain about it because at least we have a police force?
If you give people extra power they need to he held to higher standards of responsibly otherwise that power can corrupt. So yes it's okay to be angry when they get it wrong, because they have a higher risk of causing harm when they do.