I assume it's more unlimited on your phone where realistically most people wouldn't be going over that kind of limit often. Non-mobile browsing for me usually means regular sites (vs. mobile/iPhone optimized) and longer browsing, resulting in more usage. Sure, AT&T will have a hard time being able to tell what you're doing, but it's against their TOS to tether without the right plan.

(Not that I disagree with you, just pointing out why this is an issue...AT&T can and has charged people who've tethered without the right plan for the data they used..and it's not pretty.)