The glib answer is this: You have a problem. You solve it with heuristics. Now you have two problems.

If I had to guess why Apple designed it this way? Here goes: Apple can't determine what a resting hand is as an abstraction, because sometimes a resting hand should do something (e.g. the pinch gesture; one static touch + one moving finger). So app developers would each have to implement their own heuristics to decide what's "probably" an accidental touch and what's "probably" a deliberate (but sloppy) tap. Result: you won't be able to predict anymore how the iPad will behave. That's far worse than having a predictable but imperfect system.