I can verify what you said here from actual experience putting my boots on the ground on a lot of fires.
Air to air tactics is conducted on VHF AM aviation frequencies except in California where two VHF FM frequencies are assigned to each National Forest in the Pacific Southwest Region (Region 5). Each Geographical Area Coordination Center has some preassigned VHF AM aviation frequencies. The FAA has control of these frequencies and they are subject to annual review and assignment by this agency. The preassigned frequencies are typically used for initial attack only. For extended attack incidents comm folks at the National Interagency Fire Center contact the FAA to get authorization for temporary use of the frequencies they need.
I've looked at many Incident Action Plans for fire all over the country during the last three years and have noticed that on occasion a VHF FM frequency is assigned for air to air tactics. I'm guessing that either the FAA frequency assignment process was too slow or maybe somehow AM frequencies were not available.
In my experience the only time VHF AM aviation frequencies are used for air to ground purposes is when a heliport is set up. A helo manager will use an AM frequency for approach/departure control and assignment of a parking spot. Helitack trucks always have VHF AM mobiles with remote control heads mounted in various locations on the vehicle to make this work possible. I'm recently retired from the NPS in fire management and I've never seen anyone other than helitack crews carry VHF-AM radios.
