And for all the people saying offering downgrades is simply impossible:
At this moment, Apple is still signing iOS 4.1 (which came out in 2010!) for the iPhone 3GS, even though the most recent firmware is iOS 6.1.3! Thus, downgrading an iPhone 3GS from iOS 6.1.3 to 4.1 is perfectly possible (and can be done through iTunes)! There is NO reason Apple couldn't do the same for iOS 7 to iOS 6 for iPhone 4, 4S and 5. Oh, maybe there is: they want to limit customer choice and prevent jailbreaking. Why do people accept Apple's lies? Apple could enable iOS downgrades with the flip of a switch (it's literally THAT simple). Go and read some stuff about SHSH blobs and Aptickets, Apple CAN make downgrades possible. PLEASE, don't start whining about how YOU don't need or want downgrades to be possible: it wouldn't hurt you in ANY way if downgrades were possible. If you're worried about activation lock, there are more ways to circumvent it apart from jailbreaking.
Yes, and one of the reasons for that as I understand it, is because jailbreaking can allow for stealing from the app store.