Almost all carriers I know require data plan to be purchased when buying a smart phone. Their justification is that purchasing a higher plan allows them to subsidize the cost of the smart phones. If you were to buy Iphone w/o those contracts it could cost you 500 bucks. That's why it is perfectly legal and not without justification.

Now, if you want to circumvent the mandatory data plan, you might be able to get a smart phone from outside, unlock it and use it in AT&T account, as long as the phone doesn't have AT&T IMEI code (which helps them identify the phone). There might be other ways to do it too. You will end up paying more for the phone this way but will avoid monthly data charges.