This is putting the cart before the horse. Competition is what drives down prices. When companies aren't allowed to zero rate content then they're all offering more or less the same product so they have to compete with each other on price.
Also keep in mind that zero rating is itself an explicit admission that network capacity and overhead aren't factors in the price. The whole deal is that the wireless company lets customers on those plans use unlimited data at no extra charge as long as it's for zero rated content. Allowing customers at that same price point to use that same unlimited data without arbitrary restrictions would ultimately be just as profitable.
Also keep in mind that zero rating is itself an explicit admission that network capacity and overhead aren't factors in the price. The whole deal is that the wireless company lets customers on those plans use unlimited data at no extra charge as long as it's for zero rated content. Allowing customers at that same price point to use that same unlimited data without arbitrary restrictions would ultimately be just as profitable.