Let's say you have a network that has connections in Chicago, Nashville, and Atlanta. But you have no connections thst you own in New York.  You have a customer who wants to connect to a server in New York, so you need to pass that traffic off to my company that has connections in New York, Nashville, and Dallas.

My customer in Dallas wants to connect to a server in Chicago - but I can't get them all the way.  I can get them to Nashville (where we both have connections) but then they have to go to your network.

Well, to make things work smoothly, networks like yours and mine set up peering arrangements to make these types of handoffs faster (all our customers get better service and we have less problems to deal with) and usually that's a simple handshake type deal, it really makes no sense for us to monitor the usage and bill each other, since 99.9% of the time the traffic is going to be roughly equal and therefore we're going to end up cancelling out each other's bill, so it would really just be making busy work for no real gain.  Pretty much all the major providers do this, it is part of how the network is global, usable and fast.

Now, Netflix comes along, and they have customers in Chicago, New York, Atlanta, Dallas and Nashville, so they want us to peer with them - and that will improve things for the customers we have in those cities, but unlike a traditional peering arrangement, the mutual benefit basically goes out the window, because unlike the agreement and peering me and you have setup, Netflix pretty much just pushes data downstream (and they push a metric shit ton of it).  Nothing is going back up the pipe to offset our additional costs or improve overall connectivity etc.

So, me and you look at the situation and say "Hey, these fuckers are basically getting a free ride by abusing the system we've spent ~30 years as engineers making work.  Maybe they should help spread some of that cost around since between 20-30% of the traffic going through our network is theirs.". So we go to Netflix and point out our concerns and say, "Hey, we figure with our bi-directional peers, we're spending X and getting Y in return.  With your peering, we're spending X and getting... basically nothing. Maybe if you pay us an equal amount the Y would get with a traditional peering setup, we can keep doing this, otherwise, we're not going let you put this server in our data center and pay for the electricity and maintenance etc."

Netflix turns around and screams to their customer base, "These two fuckers are screwing you over!  They expect us to pay for bandwidth both ways!  This goes against the historical agreements networks are built on.  And you know what?  They expect us to pay so much, that we might have to raise all of your bills $0.05 per month to cover our added costs! Make government force them to not charge us!"