It's all tied to the subsidized purchases of hardware
Let's face it - if your company paid hundreds to put a phone into someone's hands you'd insist on charging them a monthly data charge for each device too. And I can see the advantages for those who cannot lash out the cash to pay for the device of their choice outright.
What we should have is a two tier system whereby the purchase of a subsidized phone should lock you into a contract and potentially forgo a single united data plan - but if you purchase a phone for full freight - you don't get locked in and a single data plan is an option for you - and an inducement to stay with your Carrier.
My Galaxy Tab is on a pay as you go plan on AT&T, and frankly it's demented that I have an unlimited data plan for my phone on the same network. Particularly since AT&T isn't subsidizing the purchase of the Tab (which I bought off ebay for $400 - thank you very much!)