Apple doesn't want people jailbreaking. Not even a little bit it. If you don't mind reading through 45 pages of legalese, you'll see why. It really boils down to these 13 reasons:
Crashes & instability
Malfunctioning & safety
Invasion of privacy
Exposing children to age-inappropriate content
Viruses & malware
Inability to update software
Cellular network impact
Piracy of developers’ applications
Instability of developers’ applications
Increased support burden
Developer relationships
