I'm no fan of monopolistic ISPs, but it is true that when you have a peering point with unbalanced traffic flows, the side sending more traffic is supposed to pay the side receiving more traffic.
Such an arrangement with a consumer ISP is so preposterous I have to assume that Level3 and Verizon never expected flows to be symmetric.
Level3...isn't paying for that difference.
Right, because Verizon's customers pay for the difference.