The point is that US carriers see tethering as a 'business-user only' kind of thing, and charge appropriately. With the $10-$30/month plans, all you're supposed to do is internet activities on the handset itself. The other kicker is that most carriers remove the built-in email functions from phones as well. So you're basically left with crummy internet portals to carrier-specific email, and the built-in usually-terrible web browser. That's about it.

In the carrier's eyes, paying the $10-$30/month data plan really just enables you to spend more money by buying their ring tones or subscribing to 'premium' services (for another $5-$10/month) that let you stream TV or radio. A user watching hours of MobiTV streamed on their handset is perfectly fine, but tethering to another device to check your email is not. That's just the way it is here, and it's stupid.

It's nothing new: the cell phone business model sucks in the US.