As far as I understand it,
Due to the open nature of the habitat and extensive road systems 90% of the harvest on the east side of the state was spikes already (any visible antler was legal) and few bulls survived their first few years. To increase bull to cow ratios, and allow more mature bulls to survive the spike only regulation went into effect, effectively increasing bull/cow ratios, and increasing branched bull (mature) numbers.
The ability to count Elk on the east-side by aerial survey and counts at winter feed areas, combined with harvest statistics provide the WDFW with the information to manipulate herd numbers by increasing or reducing permits while maintaining "opportunity" for non-permit holders. (spike hunters)
On the West-side...
They do not have the ability to get reliable counts from the air due to terrain and habitat, or winter feeding, so coming up with actual herd composition is dependent on harvest reports and a formula they developed to estimate (guess) on numbers using radio telemetry and fixed point observation counts.
The restriction provides enough of a level of bull escapement to meet post-hunt bull objectives and allowing enough calf recruitment to maintain numbers.
The focus is on harvestable bulls, not "trophy"... as you are probably aware, even the West-side "quality" tags are all about numbers, not antlers.
