The objective is to offset maintenance costs. Read the feasibility study- I-80 has more semi-trucks than cars on it. It's the only significant stretch of freeway in the nation with that issue.
The point here is to make money off interstate trucks, which is a great idea as these interstate trucks cost an absurd amount in maintenance costs, as trucks wear freeway surface down way faster. Since Wyoming has among the lowest gas taxes in the nation, very little of that truck money ends up coming back to Wyoming. Wyoming certainly isn't going to raise the tax on diesel to exorbitant levels, because lets face it, a very high percentage of (relative to other states) locals have diesel trucks, and that impact would be statewide, not just across what looks like Sweetwater and Carbon counties.
The state of Wyoming is rightfully pissed that it's paying for maintenance for California trucks to go to Chicago without the state of Wyoming seeing any benefit. Because nobody lives in Wyoming, all the traffic is heading for other states and Wyoming is paying for these vehicles to do that. That's not fair to Wyoming.
As far as I-70 is concerned- to go to Salt Lake, Boise, San Francisco, or Oregon from Chicago and the northeast, I-70 or I-90 would take between 4 and 10 extra hours. Most people will pay $10 for a 4 hour savings. As far as trucks- if it can scare them all away with a $100 toll it's happy to do so. No trucks would cut maintenance costs way, way down and the state of Wyoming would be better off financially.