Prevent cell phone companies from forcing data plans on consumers when they buy smart phones at full price from 3rdparty

When I bought an unlocked( I phone) from the Apple store at full price & inserted my sim card. AT&T forced a data plan of $30 on my bill. I called and complained & informed an AT&T agent that I do not want a data plan they told me it is not an option. Once you use a smart phone you have to get a data plan whether you buy the phone from AT&T or not & whether you are under contract or out of it.
This is monopoly & they do not have the right to do this. We should be able to use whatever phone we want without forcing a data plan on consumers. I understand when you buy the phone at a discounted price from the phone company they have the right to force a data plan on you. But under what law does AT&T think that they can force data plans on everybody who owns an unlocked full price smart phone.