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.
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.
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.