Seems like this is thanks to the open applications rule of the 700MHz spectrum auction, Block C[1], that Google lobbied to include and then bid to ensure that it would apply. That spectrum was bought by Verizon for the contiguous 48 states.
I guess we can hope that competitive pressure will encourage AT&T to offer Wi-Fi tethering at no additional charge. My guess is that both carriers will raise their base rate and offer it "free".
