It has been tried but the laws protecting police were broadly written before the supreme court invented qualified immunity. When every court case ends with either 'well he said he feared for his life when confronting this sitting/fleeing subject so it was a good shoot' or 'well this is clearly a violation of your rights but the courts haven't ruled that in this exact situation before so we can't expect the cop to have realized that' is it surprising that people are starting to look for other avenues to express their disapproval?