The assumption, as best I can tell, is the same that drives carriers to charge $20/mo tethering fees for using smartphone data plans with a laptop. Basically, they don't expect you to use very much of your monthly plan.
The ipad+mifi deal from Verizon is another good example. If you want just a mifi (for, say, a laptop or an existing ipad), you pay $260 + $40/mo (contract) for 250MB or $60/mo for 5GB. If you buy it with an ipad, you pay only $130 for the mifi device and get the option to buy month-to-month $20 for 1GB, $30 for 3 GB or $50 for 5 GB. With the right usage pattern it wouldn't take long for the ipad to pay for itself.
Frankly, dedicated computer links (via USB or wireless) tend to have pretty lousy rates. Why? Because the carriers know these tend to be business customers (who have their companies pay for it) and they also tend to use more of their service than many smartphone users.
That said, provided they have the coverage you want, there are good alternatives to the standard ATT/Verizon choices. Virgin Mobile sells a mifi for $200 from Walmart with a $20/mo prepaid 1GB plan (if you buy direct from VM, it's cheaper but you only have the choice between $10/100MB or $40/unlimited). It uses Sprint's network (actually, Sprint bought Virgin Mobile USA last year).