Apple has been battling against the jailbreak scene for nearly 4 years. Every time hackers find an exploit, Apple is right there to release a software update to patch it. But boot-level exploits, like GeoHot's Limera1n, are much harder to fix.
In fact, Limera1n continues to be used for iOS 5 jailbreaks on pre-A5 devices. So, Apple had to think outside the box with their latest mobile software update. They had to come up with something to try and slow down the hackers. And they did...