It's to avoid using up RF channels unnecessarily. Lots of sites may have coverage overlap, but someone defines the allowable site or sites for talkgroups. It seems IWN/WSP is still tweaking the rules as they go along, obviously it's complicated with the number of sites and TGs and learning about real world coverage.
The VHF sites usually have only 3-4 voice frequencies available. A site in the middle of King Co could be set to allow WSP Dist 2 area talkgroups but block Dist 1 and 7; even though somebody in Pierce Co has the RF path to affiliate with that site, it would tie up a limited resource when that same PierceCo radio could probably find a more local site.
In your example, I'll bet the Spokane trooper's radio will work fine on the KingCo talkgroups once it's west of the crest - they can program rules by radio ID but what I see is just based on talkgroup selection.
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