I've had this happen to me. I agree it's presumptuous and toeing the line but just so you know I've always been able call them and have the data plan turned off for good on that line.

One of the arguments I make with them is that if I have kids on my family plan who go off to school and pop their feature phone SIM card into their friends smartphone, I can't be expected to deal with that headache - and if AT&T isn't going to recognize that the person who pays the bill should have the final say in the matter, and they don't offer basic parental controls, I'll take my business elsewhere.

It's worked at least three times on three different lines off the top of my head.