And how do you suppose they will know this? Does anyone really think that ATT cares that much about the few individuals who purchased the Netshare application.
AT&T has busted people tethering on the cheaper data plans before, so it appears that they have some way of "knowing this". If you're really interested in the details, you can search around over on the AT&T forums on HowardForums.com
All the tethering would be doing is displaying the web pages on their laptop instead of their iPhone browser. The 3G connection is the same speed. All it does is display the web pages on the computer instead of the phone.
It's not the same thing at all.
If I put you at a tethered laptop and let you use it like you normally would, you'd use way, way more data than you would during the same time spent with an iPhone.
Look at YouTube. The experience on a laptop is totally difference than the optimized one for the iPhone. First off, the iPhone will only show you videos compressed with h.264. Second off, the iPhone snags all of the info about the featured videos, most popular videos, etc via a tiny XML file, not the huge web page you see when you visit with your laptop.
And then take the laptop itself. Virtually every Windows box out there is set to automatically download updates, which means while you're in YouTube, your OS can be downloading patches in the background.
The iPhone, from what I've seen, doesn't do data in the background. If you're downloading new emails and then switch to Safari, the email program stops/pauses. Your laptop's not going to do that.
There's soooo much more that a laptop can do with data that's not allowed on the iPhone, like downloading music, updating programs, sending emails with large attachments, having multiple tabs in your browser simultaneously downloading pages, and even bittorrenting/p2p.
Even if tethering was only somehow limited to webpages, the fact that the computer is going to download all of those stupid flash ads on most popular pages (that the iPhone ignores) puts it as using my data than the iPhone.