Netflix pays for Netfix's bandwith, I pay for my ISPs bandwith, and w/o Net Neutrality, the ISP charges Netflix again for transport across the ISPs network.
Without a rule saying you can't charge for the same thing twice, there's only law, convention, or competition to prevent it from happening. Since the ISP industry is famously uncompetitive, we're left with law and convention. Now, just convention.