That's exactly right.
I have the $30 data plan on my phone and the $60 laptop connect plan (5GB limit) on my MBP, and my usage on my $60 plan is four times what it is on my phone. And I'll bet that's pretty typical.
I think if people want to tether, then AT&T should offer a $60/month data plan for the iPhone that includes that feature.
The one-size-fits-all model is very broken if the occasional tethering users were forced to pay $60 a month. It is probably broken anyway though.
Personally, I'm not going to suck down four times more unlimited bits. I find it pretty messed up that if I need to ssh into a server using a full sized keyboard and bigger screen while out and about, I need to either wait til I can find a wifi spot or pop my sim into a crappy Razr to tether. Even with the crappy Razr, it is still not kosher with AT&T. Ultimately, data is data, and if I never come anywhere close to the 5GB cap, I don't see why it would ever matter to AT&T, other than sheer greed.