I think you are over simplifying things, or at least, I hope so. Just because the evidence is admisable, does not mean that the cops are free to bust in. After all there are still other punishments for violating policy, that dont involve letting criminals walk free. Its up to the departments to write comprehensive policies to cover that.
After all, how often would barging into a place without knocking be a usefull tactic? Sure, in some cases, but I'm sure the officers are well aware of the risk of getting shot.