I've never understood why this is - especially in a case like this, where the sending side is sending because (the customers of) the receiving side explicitly asked for it.
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.
That's true for hot-potato routing, but CDNs generally use cold-potato routing.
As for monetization, both Netflix and Comcast are getting paid by their respective customers.
What it comes down to is Comcast has no incentive to ensure its routes to various Internet transit providers are GOOD if they're not monetizing them. Why should Comcast be treated differently than any other transit provider and not allowed to monetize its transit services? Netflix wasn't paying Comcast, so Comcast had no responsibility to help Netflix make its own service better when there were other ways Netflix could have alleviated the bandwidth situation (e.g. buy transit through another company).
Customers pay Comcast with money to establish those links. If that's not "monetizing", what on earth is?
Comcast will get paid by their customers as long as the routes they pass traffic through are 'good enough'. They have a (near) monopoly on the last mile in many markets, so 'good enough' can mean both 'barely working' and 'better than any other option you've got' at the same time.
So, you're right, but so is the parent: Comcast has no financial incentive to provide any more than just enough bandwidth to keep you from calling the support line to complain or cancelling your contract in frustration.
Where's the rest of customer revenue going, if not towards network upgrades? Well, let's just say the Comcast Center (http://archrecord.construction.com/projects/bts/archives/off...) ain't paying for itself...
Last time I checked ISPs where heavily monetizing packet receiving to the tune of 5-10x what netflix monetizes from me. For typically slow, shitty service bundled with cable TV or phone I do not want.
Edited to add: It's certainly true they are not rewarded for the marginal packet, but then neither is Netflix (given their subscription based model).