The assumption is that if you're sending traffic to a customer, you are being paid for that activity through subscription fees, ads, donations, etc. That packet costs the receiver some minuscule amount to process. Even though it was requested by the ultimate receiver, the ultimate sender is only sending the packet because they make money from doing so. The middlemen only pass on the packet because their costs are covered and they make a small amount of profit. This setup also ensures that if the packets aren't being monetized in some way (i.e. if it's a DDoS) that there's a financial incentive to stop them.