Contradicting me doesn't make you right. And you most assuredly are wrong.
<3 People posting with no information calling me wrong.
Data you use while tethered is not included at all. Thus, it is all overage. It's similar to sending SMS messages on an account that doesn't include them.
Find me a place that says there's a difference between tethered data and normal data. Is it in your bill? No, it's not. AT&T treats data as data as data as data. It makes no differentiation in your bill on how you spent it.
And it's not at all similar to using SMS messages on an account that doesn't include them, good try with your simile there. A better comparison (if you were actually right) would be using MMS on an unlimited SMS plan where MMS was not included -- but that's still pushing it.