It seems to me they were "federalized" to help in state surveillance and extermination actions, but I don't want to jump to conclusions. Sources and book recommendations would be nice.
See the thread What was the criminal justice system like in Nazi Germany?, feat. the work of /u/idenpoelchau, /u/estherke, and /u/commiespaceinvader in direct response to your question. Several of the answers provide sources in German and English.