The national brands and store brands are always next to each other on the same shelf. So there isn't any problem that needs solving. Also brands already pay for promotional positioning (like the ends of aisles), and when the store puts its own brands in those spots it loses out on those payments, and so is effectively paying equally.
So the field doesn't need any evening out there.
If grocery stores ever started putting the name-brand stuff in a back room that took twice as long to get to, then it might become an issue, but as of right now it's just not.