Would Apple ever approve a programming app for the iPhone or iPad? It's capable of running such things, but it would mean less control for Apple. To allow users to write new apps right on the phone is the very definition of "arbitrary code", which is a big no-no in security terms. But if programming itself is a security risk, then it's a risk everywhere, not just on the phone. Who will write the new apps when nobody is allowed to program any more? You can't win a lock-down battle as well as embrace the developer community.
Mokalus of Borg
PS - At some point, someone will want to do something you haven't thought of.
PPS - And then you're holding back innovation.