The problem is that the law often isn't clear. You have assault, battery, manslaughter, murder, unlawful detainment, and similar laws on the books, all with varying requirements and all with various carveouts for law enforcement, self-defense, etc. They tend to be written in broad language because real life is complicated. In the end, the law in this area is generally in the form of case law created by courts in response to novel situations. The courts have to make judgement calls. In my experience, LE agencies make a lot of effort to ensure their policies conform to existing caselaw and cover the gaps in it. Sometimes they get it wrong. But more often than not, when a policy is particularly egregious, the underlying law is to blame.

In the end, police have broad authority to use reasonable force to effect lawful arrests and searches. And most people don't understand just how broad that authority is under the law.

Which is why the cases in which the police harm or injure people should be brought before the court. Not decided by a prosecutor or grand jury, those entities shouldn't be deciding what is lawful.