For example DC Police may have confidential informants who would be best to use Signal because that isn't unusual. But the people there are communicating need to retain the communication.
This is the fundamental problem that end-to-end encryption doesn’t solve, right? If the person on the other end is malicious or really dumb they can still leak your messages.
E2EE’s biggest use case is preventing the government from reading your messages. If you are messaging the government (or are in the government) then this isn’t relevant.