Why are jailbroken apps not allowed on the App store?

For Iphones some of the apps that one can obtain with a jailbroken iphone are actually quite useful. For example there's a numerous amounts of apps that give the ability to lock folders so you can only open the folders with a password. This could be useful for many people. There's also another app that allows you to send a text message without actually going to the text message app. This can save a lot of time.

Why aren't these apps allowed on the app store? I don't see anything harmful about them. In fact they make the iphone a lot better.

$$$$$$$$$$$$$$$$$$$$$$ oh and