Many/most multi-site trunk systems behave that way. The term is a bit of a misnomer but to someone who is new to trunk systems it describes the concept adequately. Simply put, if radio 12345 logs in to the system, and is tuned to talkgroup 56789, the system is instructed to provide resources for that talkgroup to the tower that 12345 has affiliated to. That uses up a voice channel whenever that talkgroup is active, of course, and whatever linking pathway is necessary to bring the audio from the originating tower to the tower 12345 is on. If you have 30 agencies with their own different talkgroups and all their members in different parts of the province "dragging" their voice traffic across the backbone, you have a potential to saturate it with unnecessary traffic.
There's no real way to avoid the phenomenon other than, as you say, staying off the channel for area X when you are in area Y (or even area B). Or spending gobs of extra cash to make the backbone massive and have the max numbers of frequencies (32?) on each tower site, again, costing lots of money for hardware.
As for non-RCMP users on PACS, it is indeed more prevalent in the past few years. It only makes sense to have agencies able to talk to one another. I listened to an incident this afternoon where the fire/rescue crews were going out to search for/rescue someone and the RCMP had been notified, but weren't able to/didn't communicate with the fire crews. Everything had to be relayed back through dispatch centers.
You don't need AFRRCS to make that happen, either. Letters of agreement between the responder agencies to use common channels are more than sufficient. Having said that, though, AFRRCS had better have a bunch of "Interop" or "Common Events" (as the folks on the coast put it) channels set aside when they lay out the talkgroup structure.
