How much is MT F&G budget $75MM? How much do doe tags go for? $10? If you think this is about "money" you'd likely be mistaken. Without getting into the details of the budget, I really doubt that an extra $100,000+/- is really about the "money." They wouldn't even notice it was gone. If they need to sell doe tags to keep afloat, they're fugged.
You are likely correct in that it is not about the money specifically raised from these tags. I believe it is somewhat based on human perception of overall herd health should they declare mule deer does to be off limits to boost struggling populations. The ripple down from the that could be significant.
The largest factor IMO is that the management area is far to large. I am born and raised in region 7 and spend a significant amount of time there each year. Mule deer populations vary significantly when one travels from Broadus to Miles City to Jordan, back east to Wibaux then north to Sidney. The allotted tags can be used through that entire area and then some. What is to stop 8000 does from being shot in the Custer, or Prairie County, Richland County, etc? Nothing as far as I can tell.
When I've asked biologists that question, it was stated that they manage under the assumption that hunters will naturally distribute to areas with higher population densities of the game animal of choice. Limited hunter access doesn't allow for that theory to be feasible.