Let's say Netflix is sending 10Gbit of traffic to Comcast's customers. If they have a direct peer, Netflix doesn't have to pay to send that data over transit, so they're benefiting the most, right? Well no, because without that direct peer the customers still want that traffic and now they've got 10gbit of unbalanced traffic coming through one of their other providers, likely their transit provider. Unbalanced traffic is bad because you pay for transit at scale in a fairly symmetric fashion.

So, ISPs have to pay per port to exchanges, usually per port plus bandwidth for transit, and one off hardware only for a settlement free direct peer. If you'd need a full port's worth of connectivity regardless of which type of path you take, which do you think is cheapest to go through? :p

Of course the other thing is that if you're maxing out a direct peering link, and you're refusing to upgrade the agreement without a fee, you have a very convenient way of throttling the traffic from that provider without actually using an "intentional" throttle like net neutrality is supposed to prevent, plus you get to shift the blame to someone else and not take the PR hit from throttling the connections.

If you want to know what net neutrality is really worried about, go back a couple of decades and remember what the walled gardens of AOL and CompuServe were like. AOL's walled garden died because so much dial up competition made it too easy to get real unfiltered and untiered internet. Cable and radio based internet doesn't have that competition space in most places, that's why tiered internet and throttling is so dangerous now.