Apple claims to have a walled garden to maintain a high quality of applications in the App Store and to maintain the image of the iPhone.
I’m not sure that’s a good idea – there are arguments both for and against a walled garden, my personal opinion that a walled garden that has an option to disable it if you know what you’re doing (jailbreaking doesn’t count – something like WebOS’s “webos20090606” or Maemo’s Red Pill mode or rootsh do count) is what’s best. Keeps the users that don’t know what they’re doing safe (and even allows much tighter restrictions on the quality of content,) but lets people run unapproved apps without having to run afoul of various agreements.
But, my opinions on walled gardens notwithstanding, Apple does claim they do it to maintain high quality.
So, why are they approving completely useless applications, such as a mirror app, that just take up space, and reduce the signal to noise ratio of the App Store?
Source: App Rejections