> it can't possibly cost them anything.
A server in New York wants to stream data to a user in California. Someone has to pay for a nationwide fiber network to move the traffic. Both Cogent and Verizon have presence in New York and California. Who carries the packets from New York to California? Very often, that's what peering disputes come down to.
In general, neither party wants to carry the traffic across the country. Cogent wants to instantly hand the traffic to Verizon right there in the same building where they got the traffic from Netflix, so they don't have to carry the packets more than 100 meters. That's reasonable to them - they are delivering the packets to the company they are addressed to. Verizon would want to receive California packets in California. When Cogent is charging Netflix for transit, it's reasonable for Verizon to ask Cogent to provide that transit to California. Both have reasonable positions. They'll negotiate a mutually acceptable arrangement after first staking out their starting positions.