Fallacious assumption: you assume that just because I choose to have my data ported to my ipad that I will use more data - wrong. You also assume (perhaps not) that it costs more to tether (it doesn't, and only uses a bit more battery life from the tethering device, so I'm told). I use MyWi tethering exclusively to get weather nexrad images just before flights. I can get the same weather updates on my iphone4 that I do on my ipad2, but it is more convenient to use the ipad, due to the larger display. On those occasional times when I want it to display on the ipad, why should I pay more than I do on the iphone for data that is merely re-routed from the iphone to the ipad? Now, if I was downloading movies or a data hog on the larger format device (a valid point), it would make sense, and that supposedly is why ATT and Verizon want separate contracts on each device.
However, ATT does not provide a pay-as-you-go option for tethering. It is a capped monthly plan that is too expensive for the occasional user like myself, and too expensive for the intensive data downloader, who would easily exceed the plan limits, so its only reason for being it to extract as much money from as many folks as possible). Really...who is nickel and diming here.
It is really no trouble to jailbreak your phone and use MyWi (the only reason I need to jailbreak it), so why criticize those who do it for a legitimate reason, primarly based on principal and convenience. Don't get me started on how ATT has tried to screw us over time and time again over the years. No contest here, I have spent many hours on the phone getting fraudulent charges reversed from ATT over the years to shed a tear over my inconsequential tethering.
I'm not making any assumptions about what you do with tethering. OTOH, I can tell you with high probability that the overall data usage across all users is going to be higher as you add devices to a plan which, in aggragate, is happening when tethering is enabled. Short of a true pay-per-megabyte plan (there have been huge outcries in this forum over such an idea), I can tell you AT&T isn't going to make you or anyone else a custom plan.
Most important, you identified the major reason why this unlimited tethering in a plan would raise rates.... people are much more likely to watch data-intense movies on an iPad as opposed to an iPhone. Ultimately, more data costs more.