But aside that, I think that beyond the "we can charge you for data because you can", the per-data-transferred costs come from the overprovisioning ISPs do. The fraction of the advertised to actually supported bandwidth per user they can get away with is dependent on average amount of traffic users generate.
How do data caps help with that though? People are still going to use most of their internet during peek hours, because...well, those are the times people use the internet the most, by definition.
It's like trying to combat rush hour traffic problems by limiting the number of times people can use a highway. People are still going to use it during rush hour, they just are going to avoid using the highway during non-rush hour periods, which accomplishes nothing.