Well, this analogy is not useful. The point is that it is okay for some devices to not run arbitrary code. There is no reason the iPad really needs to do that,

Well, this analogy is not useful. The point is that it is okay for some devices to not run arbitrary code. There is no reason the iPad really needs to do that, other alternatives are available and most people do not need or want them. Building things requires tradeoffs, and the iPads make a reasonable tradeoff here.