A friend of mine who works for Apple told me (a year or so ago) that the reason for this is because ATT (and similar companies in other countries) wants to discourage tethering due to the fact that they don't have the infrastructure to accommodate lots of high bandwidth connections that you use when you tether. They were gambling on the fact that you need to do a lot more "work" at the moment still to browse/etc. on your phone. But this was before multitasking -- I can only bet they were throwing their hands up in dismay when multitasking came in :)
I can understand this idea when it's an unlimited plan: if you tether, you're likely to use a lot more bandwidth than with just your phone. So, if you tether, you'll impact the infrastructure.
But since they're limiting your bandwidth already, I'm not so sure why it matters if you use it from a laptop or from your phone.
In the end, I know why they're doing it: because they can, and because they want to limit the number of people who do it. It just bothers me because it doesn't make sense technically.
