The assumption is that since those devices lack (convenient) input devices, the user has a hard time controlling them and therefore they get a free run.
In a unix system, a runaway program is very close to a kill command.
It's so ingrained int the ecosystem that it's actually one of the easiest command to give, in any UI provided: C-c in the terminal, C-a K (or close button) in window managers, etc.
But on handheld computers, there's no key, no button to kill a program. The best you can do on iOS is a double-click on the home button, then a long-press on the icons, and then a click on the delete button on the icon. On Android it's similarly convoluted.
This is also the reason I guess, why originally no programming (or scripting) system is provided on those computers.
But it is more from a lack of imagination on the part of Apple and Android, than a real limitation of the input devices. You have to remember that those devices and their software is not designed by programmers, but by fashion designers. The purpose is to make them beautiful, not practical, so they may sell a lot of them thru pure peer pressure and keeping-up-with-the-joneses. My golden iPhone is better than your silver iPhone.
So basically, there's space for whole new OSes running on those devices. Probably easier done on Android where the hardware documentation might be obtained more easily, but you could also replace iOS on rooted Apple devices.
