I like how it mentions that they "allow" you to use wifi at no additional charge.
Imo, the only way they could justify charging for tether is if you then get unlimited. To go over 4gigs you pretty much have to be tethering.
I tether to my nook, and even then, I'm not doing anything I can't do on my phone, its just on a bigger screen.
I think, if they're data capping, they should allow tether at no charge, as long as you don't go over the cap. Hell, this could net them more money, in that charges over the cap are outrageous, and you're more likely to go over the cap whilst tethering.