This is a fair point, but Apple addled the iPad with those limitations from its inception. The stated reason back then was that it wasn't powerful enough - which I disagreed with even then. If it had been able to boot Linux or OS X, I would have snapped one up. Now the only real difference is form factor, pricing discrimination, and touch screen (although I still believe that on some level it's a point of embarrassment for Apple that their desktop OS is completely unusable by touch, hence their endless reluctance to bring a touch-enabled Mac laptop to market. However that may be the factor that saves Mac OS from going completely down the consumer road of iOS).

So you're right that these categories of devices and capabilities are completely artificially imposed at this point, but the question is, why have an iPad at all?