So we get these wonky billing schemes that the ISP can't really defend because they're not about to come out and say that their customers aren't rational enough to handle a more sensible scheme.
To be fair, I'm sure the fact that it kneecaps Netflix doesn't hurt their enthusiasm for usage caps, and residential ISPs are overpriced in general, but there is an element of customer behavior which pushes ISPs in this direction.
I don't want to think about what time it is when I go on Facebook or whatever, because I have more important things to think about. There is nothing irrational about that (though the premise that increased rationality is something to be desired in this context is deeply questionable to begin with and maybe slightly creepy.)
When an ISP can offer a plan that requires me to think as little about it as possible, that simplicity is a valuable service.