Netflix is saying "your roads do not meet the needs of your residents. They're asking for a lot of trucks but your roads aren't large enough". Although this metaphor is getting rather strained because cities don't charge their residents for varying levels of road access.

So without the metaphor, Netflix is telling Comcast that their links do not have the capacity to serve the needs of their (Comcast's) customers. This isn't something that Netflix should even need to tell Comcast; Comcast already knows it. In fact, not being able to meet their obligations to their customers was an intentional decision on Comcast's part. They assumed that their customers wouldn't actually use all the bandwidth they are paying for. But with the rise of video streaming in general, and Netflix in particular, Comcast's customers are using more and more of their bought and paid for bandwidth.

That's the issue right there. Comcast intentionally oversold their capacity (or intentionally under-built their infrastructure, whichever way you want to look at it), and it's now coming to bite Comcast in the ass. But instead of admitting that they did this and working to build out their infrastructure, which would cut into their obscene profits, they're trying to blame Netflix instead and demand that Netflix pay. In short, they're trying to double-dip.

It's a simple fact that if Comcast's customers can't reach Netflix (but customers of other ISPs can), then Comcast has sold a false bill of goods to their customers. They haven't sold them "access to all websites that are willing to pay for the privilege of talking to Comcast". They've sold them access to the internet. An internet that includes Netflix.