said by heh :
Because some moron would buy a smartphone without a data plan, and get a bill for $12301789. Then AT&T would get roasted for not forcing the customers into a data plan.
You are right - that is the reason. But with almost all smartphones now supporting WiFi, the providers should let people buy voice only plans and disable data access over the 3G networks. All the smartphones with WiFi that I have seen have an option to turn OFF data access thru the cell network.
As a compromise, I would let them refuse to provide this voice only plan option to SUBSIDIZED smartphones. That is, bring your own unsubsidized full cost smartphone to the deal if you want a voice only plan.