Untrue. Sorry.
Might want to try reading the Terms of Service that you (and anyone else using AT&T) agreed to.
"An eligible data plan is required for certain devices, including iPhones and other designated smartphones. [...] If it is determined that you are using an iPhone or other designated smartphone without an eligible data plan, AT&T reserves the right to add an eligible data plan to your account and bill you the appropriate monthly fee."
There's not a single word in there about "data plan only required when purchased with 2 year contract". If you don't want to believe the ToS, feel free to search this forum for the various posts from people who got an iPhone from a friend/eBay/etc and got slapped with a data plan.