We all know how _that_ played out...
It just so happens though that the "talking" it is doing is transcribing bits to/from the internet connection of the phone.
The App itself is incapable of "tethering" without the piece installed on your computer that translates what the App is sending into a normal looking ethernet device.
Think of it this way... if I made an app that was called USBTwitter... and all it did was read bits over USB and posted them to your twitter account... then I made an executable that you ran on your Mac that sent text bits over USB... you would then be able to post to twitter _through_ your phone (ie tether).
This is the very same thing... but of course more general.
You honestly can't stop this without disallowing apps to use the USB port on the phone (or trying to explicitly ban it in the developer terms... but it would be hard because there would be plenty of "non-tethering" applications that would want to talk over USB and send bits over the phone's net connection simultaneously...)
It will be interesting to see what Apple does here..."
http://forums.macrumors.com/showthread.php?p=13929912#post13...
I only need tethering once a year. This iTether app would have been handy last week.