Also because any expectation of privacy you have after hearing "This call may be recorded" is per se unreasonable, so the Fourth Amendment has no applicability here.
Sure but once they start moving into the pen register side of things to connect the dots between the folks on the other side of a prison phone call and the second level of connections they create you start to get into some gray-er areas. It becomes easier to dig through a trash can to find cause for a warrant than to use the call + pen register data.
