The problem is that is that for people who live along these roads, it’s literally impossible to go anywhere (apart from your neighbor’s house) without crossing into another state. I actually found an example of what I’m talking about very easily just by looking at a random state boarder on Google Maps – the aptly named S. Stateline Road on the Washington-Idaho boarder. Following the road from north to south, the road starts in Idaho, appears to run right on the state line for a bit, crosses into Washington, then back into Idaho, then on the line again, then back into Idaho where it ends in a dead end. For people on the Idaho side between the dead end and the border (and I see several houses there on Satellite View), you can’t go anywhere without crossing the border into Washington. There are other roads that branch off of Stateline Road into Idaho, but they’re all dead end, too – the only way out is through Washington.
California’s agriculture checks solve that by putting them 30-40 miles inside the border on major roads.
Thinking that “we over here” can relax and let down our guard because the infection is only spreading among “them over there” is living in a fool’s paradise. If you’re going to be enforcing mask and distancing rules then it really doesn’t matter where someone has been. If you’re not enforcing those rules, then you’re welcoming the infection with open arms, no matter how stringently you try to keep “them” out. All you achieve with an interstate travel ban is interstate bad feelings, not what we need in this country.