That downward compatibility stuff wouldn't be neccessary if Apple would reject the apps (while in approval process) that don't follow their development guidelines. Why do they have an approval process, after all?
They catch more things than otherwise, but stuff can still get through. Of course, it isn't impenetrable to woes. In fact, in recent news, "tainted" apps managed to get onto the AppStore b/c developers used XcodeGhost, a Chinese modified version of Xcode that injects malicious stuff in dev's works. Techincal folks are saying there just simply wasn't way Apple app reviewers could've caught that.
