Allowing it would create security issues; nothing to stop the app from showing a fake system bar that appears to let you out of the app, but actually keeps you

Allowing it would create security issues; nothing to stop the app from showing a fake system bar that appears to let you out of the app, but actually keeps you in and snoops information. And having no hardkeys means there's no 'safety hatch' that you know will get you out of the app (like Flash's "esc to close" warning)