It's an old & interesting debate about how data service ought to be billed. If 26 customers pay for 50MB/s service, most use 2MB/s, one uses 50MB/s, and the uplink is sparsely provisioned at 100MB/s, what is fair? One customer is using half the capacity of the link but only paying 1/26th of the costs, while the others are using 1/50th of the capacity and paying 1/26th the cost. Many might argue the one customer should be paying more.
They could send back a whole bunch of zero bytes to make the traffic symmetrical. They save everyone's resources by not doing that, yet somehow that means they should be charged...
The direction of the traffic is meaningless. What matters is the endpoint of the connection. ISPs should be happy to peer with anyone for traffic that has their users as an endpoint.