It is illegal because it involves violating Apple's copyright on their firmware. The legal status was established long before there even was an iPhone to jailbreak.
That filing was provoked by the EFF's attempt to get it made specifically exempt from copyright protection, but their arguments didn't actually meet any of the established criteria for creating exemptions. Apple's filing, in context, was an objection to the EFF's claims mostly spent pointing this out. The EFF tries to characterize it as Apple trying to get jailbreaking made illegal, but that's not the case. If it weren't already a copyright infringement there would have been no point for the EFF to make their filing in the first place. The whole thing is a bizarre kind of legal trolling, and cringe whenever anybody mentions it because it so severely tarnished my image of the EFF.
That filing was provoked by the EFF's attempt to get it made specifically exempt from copyright protection, but their arguments didn't actually meet any of the established criteria for creating exemptions. Apple's filing, in context, was an objection to the EFF's claims mostly spent pointing this out. The EFF tries to characterize it as Apple trying to get jailbreaking made illegal, but that's not the case. If it weren't already a copyright infringement there would have been no point for the EFF to make their filing in the first place. The whole thing is a bizarre kind of legal trolling, and cringe whenever anybody mentions it because it so severely tarnished my image of the EFF.