only has the old unlimited data plans, but like the poster before me stated, ATT will throttle your data down after you surpass a certain amount. If you have LTE, you will be slowed down to 3G, if you're lucky. The idea is to frustrate you into not using anymore data, because it is so slow.
But to answer your question as to why they don't have any unlimited data plans, MONEY. ATT makes more money by not having them. Same reason why they are tossing the 2 years contracts and replacing them with the next program. It's labeled as a good thing for you, but the price you pay for the phone in the end is more than you would have paid if you brought the phone outright.