That is totally wrong. If it was true then all public safety frequencies would be blocked.
Not really.
All it takes is someone in bed with GRE to agree to a block of the repeater input ranges.
There have been reports of criminals using scanners tuned to the input side (mobile/portable xmit side) of a repeater. They just use the strong signal strength alone to determine that a LEO is near, very near.
It does not matter if the signal is encrypted or not as they are only looking at signal strengths. They could care less what is being said.
I've worked many times with a group of guys that do security for Monsanto and that is how they protected the meeting rooms from industrial espionage.
They took a fingerprint reading from our transmitters so they did not falsely shut the meeting down. They looked at RF signal strength in general. If they saw a spike not matching out fingerprints, they would go in and shut the meeting down. Criminals can do the same and evade the police even if using encryption. That trick does not work on repeater outputs as they are always the same strength.
I know this trick has been used around here at meth labs before. My source said they hooked up an alarm that triggered if the receiver picked up a signal stronger than a certain threshold. I don't know if they caught the bad guys or not but the chemicals and the scanner were retrieved along with the attached circuit that alerted the bad guys. The LEO's were also not using encryption at the time anyhow.
Of course any smart criminal would know how to build an RF detector and not even need a scanner but who said all crimnals are smart. In the case here that I know of, law enforcement was simply acting on a tip. They did not expect to arrive when things were going on.
Now I did say this was "just my suspicion" and I agree that it is unlikely plus I also agree with what Chauffeur6 said as that actually makes sense.
I recall when the FCC started making them epoxy over the band switching components on the scanner boards so one could not as easily modify a receiver to receive the cell bands through images by switching different band filters in and out of the circuit. This was added into the law a few years after the initial ECPA was enacted.