Because they already paid for access, to their own ISPs. Their ISPs paid for access to backbone ISPs, which paid (through peering or peering + money) for access

Because they already paid for access, to their own ISPs. Their ISPs paid for access to backbone ISPs, which paid (through peering or peering + money) for access to other backbones. End-user ISPs pay the backbones for access, or (in the case of the large ones) may run their own backbone services. Either way the end-user ISP has been paid by their user for access to the backbone ISPs. It's as though the Canadian trucks paid for their gas in Canada, and then get charged an import duty for it upon entering the US.