Criminal investigations 101... you just got a FYes. Suspect Representatives, for example, can better formulate lies if they know, in advance, whether they were recorded, who they were recorded with, and what they were recorded doing.
Criminal investigations 101 really.
Your saying the state can withhold evidence from the defense...ever heard of discovery
A judge would be the one to determine what evidence can be shared
I guess your knowledge of legal systems is Soviet style