That certainly is interesting.
I'm guessing that this is 1) due to a poorly designed architecture for "signaling" as well as 2) a reflection of the fact that laptop users still aren't using their laptops & tethering enough
Seriously, this does seem to point out, then, that adding tethering won't do as much harm as selling 100k more iPhones. In fact, if it converts data usage from the back and forth of smart phone usage to a stable, data-intensive by signaling-limiting method, then ATT will come out ahead by allowing tethering.