It wouldn't be hard to use short term keys which could be broadcast out one hour after use. Public would be able to watch the watchers and criminals would not be able to track police response real time.
I think this would actually be very hard, especially with the gear used by most agencies. Motorola, for example keeps everything locked down and buried behind exorbitant costs. Just a single handheld radio can cost upwards of $6,000, and re-keying the encryption among other re-programming needs can only be performed by Motorola techs.
It's much easier to just offer the media a radio that has already been programmed. This is commonplace at news agencies since they can afford it, but street journalists are frequently left in the dark, and I don't think many police agencies are too upset about that.
This would be a technical nightmare given how police radios work. However, you can submit a FOIA request for radio recordings, which basically does what you suggest but with a longer delay.
The radios do support over-the-air keying, but it is rarely used. Having physical radios come back to the shop for key refreshes provides positive control over radios, and prevents a stolen radio (which happens more often than you'd think) from listening in on a SWAT or drug operation.
ehhh, there's a bit of a difference. if i record encrypted traffic then i am granted a decryption key later, i can be certain that nothing was edited out.
It's much easier to just offer the media a radio that has already been programmed. This is commonplace at news agencies since they can afford it, but street journalists are frequently left in the dark, and I don't think many police agencies are too upset about that.