AT&T detect your phone and add data plan automatically. Recently I had big fight with their customer care. They refused to remove $20 data plan because now it is their policy if you use smartphone then you have to buy their monthly plan.
I would suggest go for MVNO like ptel or straight talk with these phones. You will save lot of money in long run
fighting with them will lead you nowhere...it will only make them force the issue more..
politely asking for it to be removed as you didn't know about would have probably worked and tell them you'll get another phone [NOT!]....in other words, play dumb and engage your csr in small talk...