In a free market the consumers hold the power. An ISP that didn't restrict Internet usage or tier what it offered would become in-demand, and would profit and succeed more than the others. In other words, the market would correct itself towards those ISPs that espoused the principles of staying neutral. If that didn't happen, then net neutrality isn't a big enough concern to drive the average consumer's purchase.
It's a simplified generalization, but take from it what you will.