Replying to The iPod Touch and the iPhone...

can both connect to Wi-Fi, surf the web on Safari, and receive emails through the internet. So why is it that AT&T charges $30 a month for the iPhone's "data" plan and with a iPod Touch its a one-time-fee sort of deal. Is the answer obvious and i just haven't read the right information on Apples and AT&Ts websites? Or is AT&T really just charging you extra money because they can and people will pay it because they want an iPhone? Much thanks to anyone that can help or explain this to me...