said by PinkyThePig:
Is it even legal to sign them up for something without their permission?
No. However, you gave permission by agreeing to the 50-million pages of the Terms of Service.
(Note, however, that the blog post makes it clear that he's off-contract.)
The problem is that the carriers charge extremely high fees for per-MB data if you do not have a data plan. Thus, if you have no data plan, forget to turn off data on the phone, and try to use data, then you could easily rack up thousands of dollars in charges.
AT&T judged it better to upset the minority of users who have the discipline to have no data plan and use Wi-Fi everywhere, rather than to upset the users who cut the data plan and then continue to use data.
There are two obvious solutions to the "stupid users" conundrum: (1) Don't charge so much for data off a data plan! (2) Disable per-byte usage if you don't buy a data plan. But, well, we know that obvious solutions are never adopted.
AT&T can force every smartphone onto a data plan, and pick up additional revenues. Or they can lower prices and/or turn off data, and forgo the revenues. Which of these two options do you think they'll pick?