Traffic balance is not the primary measure these days (from what I understand), it is just an economic decision. However, the Netflix case is interesting, because they were essentially used as a leverage tool by Cogent against the other carriers. Cogent has a long history of trying to get settlement-free peering, not meeting contract terms (whatever they are), getting dropped, and then blaming the other side. They have long wanted to be a settlement-free "tier 1" provider (which is a nebulous term, but go with it), but have generally not been. They sell bandwidth often at below-market rates in order to attract customers to leverage against the other "tier 1" providers. They saw Netflix on the rise and grabbed them, apparently selling bandwidth much cheaper than any other backbone (possibly at a loss even) in order to leverage settlement-free peering contracts out of other providers.
Any network engineering with a clue knows that you never buy bandwidth only from Cogent (or even Cogent and one other provider), because you _will_ get disconnected from somebody when Cogent gets in another peering dispute.