There is nothing baffling about it. When there is a fire, most FD people would like nothing more than to have no one there except FD, PD and EMS- no crowd of on-lookers, no "wanna-bes", and no news people to make even more people think they should be right there at the time.
I have to agree with Gary. As a fire scene photographer, I have been welcomed on scene in the majority of places that I have shot fires. That includes Boston, FDNY, Memphis, Detroit, Camden, Philadelphia and a bunch of others. My photos have been in most of the fire service journals and I always offer to send the images to anyone in the department that is interested.
What I have a bigger problem with is my own agency. In Boston, police, fire and EMS each maintain their own systems and if one was going to encrypt, it is almost a certainty that they would not share the encryption key with the other two out of concern for security. It may not be a first amedment issue and it is a privilege to be able to monitor, but if I am responding into a scene, I want to be able to hear (I feel it is my RIGHT as a co-responder) what's being said by the other agencies. More than once I've heard officers reporting active shots fired and held at a safe location, only to get the update from my dispatcher several minutes later, when I would have already driven into the scene.