However, you must remember that every single packet that Level 3 sends to the Comcast network is a packet that was requested BY A COMCAST CUSTOMER. Level 3 isn't just deciding to send a bunch of traffic over Comcasts network, they are sending the data requested by Comcast customers to that network.
This is an important distinction, and is usually part of all of these 'settlement-free' peering arrangements. In agreeing to peer with Level 3, I am sure Comcast has an agreement that they will not send any traffic to Comcast's network that is not actually destined for a Comcast customer. Now, if Level 3 were sending traffic to Comcast and expecting Comcast to route, for free, that traffic to another 3rd part network, that would NOT be kosher, and Comcast could fairly ask for either money or for Level 3 to stop sending that traffic to them.
Level 3 is ONLY sending the traffic to Comcast that Comcast is requesting; it is THEIR customers who are creating this demand for Level 3's customer's content. Comcast told their customers that for $X per month, they would get Y amount of internet bandwidth, which will in almost all cases be traffic originating from a non-Comcast network; that is just the nature of the internet. Even though Level 3 is willing to send the data that Comcast's customers want to the Comcast network for free (settlement free), Comcast is instead refusing to accept all the traffic that their own customers are requesting and demanding additional payment to provide the service they already sold and are paid for.