Cell phone operators in the US domestic market live or die based on the principles and availability of
price discrimination.
Long story short, if carriers starting charging some flavor of reasonable rates that approximate cost (such as would occur in a
perfectly competitive market whereas what the US has more closely approximates an
oligopoly) then
a majority of the high revenue corporate users who have $50/monthly data plans on their blackberries would all of a sudden be paying [perhaps] $5/month since they use all but a couple megabytes a month.
Further, like with voice, the true
cost driver of, virtually any data use, is the maximum available bandwidth (and, to varying extents, the underlying quality of service/latency on the connection). Not all phone calls use the same amount of data but they are billed the same (e.g. if you had a long phone call where you are on hold with silence for a long time, that silence is largely compressed).
The carriers realized that most phone calls use a similar amount of data per minute. Accordingly, they decided that a reasonable and intuitive proxy to charge by bandwidth is to charge by peak or off-peak minute. They understand, like with the blackberry business users, that people have a different demand curve (i.e. they are less cost conscious, on average) during business hours for a variety of reasons.
With data on the other hand, there isn't a good intuitive proxy
that makes the industry money to split up bandwidth. If carriers implemented something like peak and off-peak rates per megabyte it would decimate the money they are collecting from blackberry users, their cash cows. In fact, blackberry users are further "discriminated" against in that if they access a blackberry enterprise server, there's typically a surcharge from the carrier despite it not costing the carrier anything additional (verizon is guilty of this). This charge would also disappear with such metered usage.
The telecoms are in a tight spot though I'm not losing any sleep over it. They could get away with what they've been doing with some laughably price discrimination based plans (e.g. ~$20 for 10mb of blackberry data or $20 for 250mb of ipad data) or congress could allow some meaningful wireless
carterfone legislation to pass which would eliminate this price discrimination.
I doubt the lobbyists would allow that
:)