Nazis were able to control police because of WW2. They had complete control over Germany and Europe. You would have to listen to them. They probably would kill the police if the did not give them the role.
No one wanted to get killed from a war. They tried their best to obey the Nazis, to not get killed by them. So, they let Nazis take full control of their police duties. This is why the Nazis were taking complete control of police, from 1933 to 1939.
Hope this helps. Feel free to ask more questions on your mind.