Statistically speaking you will use more bandwidth on your laptop than on your phone. Quite simply because it's easier.
"Unlimited" data isn't really unlimited just like your unlimited cable modem at home isn't really unlimited. The carrier reserves a right to dump you as a customer and you will be dumped if you start losing them money (which is what you'd be doing if you saturated their network 24/7).
So I hear what you're saying, but this has been argued 1000 times before and you're not going to make AT&T change. It's much cheaper for them to take the PR hit and fire you.
While iPhone users consume a lot more bandwidth than other users, AT&T can sort of keep it in check thanks to the tight restrictions on the AppStore. Once they turn on tethering those restrictions go away and they'll have people using it as their main connection and all that comes with it (BitTorrent comes to mind).
Personally I'd rather pay by the megabyte--I don't want to pay $30 a month for data so I can have an iPhone. I just want to SMS and from time to time check a map. I'll let people like you subsidize their network build out.
Statistically speaking what statistics are you referring to?
More on point, though, because it's "been argued 1000 times before" are you suggesting that it's best for consumers to just shut up and take it? How about you speak out when you're unhappy and encourage others to do the same. Whether or not you're going to change an AT&T policy is not the point. There are other ends that are just as good. If apple felt that more iPhones would be sold, and at a higher margin, if they opened it up to an alternative carrier in the US market, they'd do it in a second once their initial AT&T contract expires.
Further, you can buy a data card for your laptop. It's not like nobody is using their networks to connect to the network. And my point is that they're doing it right now, this very moment, and they're blocking BT and restricting other traffic.
Their policies make it very clear that they don't intend their laptop connect cards as a users primary connection, even if their marketing doesn't.
That's why tethering plans are commonly sold with a GB limit and phone only plans are not. But really it's common sense, you're much more likely to consume bandwidth heavy stuff on your computer. Also many of the same services will use more bytes on your laptop. YouTube on the iPhone is low-quality and low-bandwidth, visit on your computer tethered to your iPhone and you're using way more bytes.
Either it's unlimited, eg use as much as you want - really! or it's not - only use basic web/email stuff on your iphone.
If it's limited (Which is what you're saying), then they should not market it as unlimited data.