How does AT&T Identify what kind of cell phone I am using?

Hello. I currently use AT&T as a cell phone carrier. I've noticed that everytime I switch my sim card into a different phone, AT&T is able to tell what phone I am using. For example. If I have my sim card in a motorola v197, at&t is able to tell that i am making calls using a motorola v197. If i have my sim card in a Blackberry curve, AT&T is able to tell im making calls with a blackberry curve. How are they able to do this? The reason why I want to know this is because AT&T charges more for their unlimited data plan if you use a PDA phone, which I think is sleazy. Why should you be charged more money just because the phone you are using is a smartphone?