I guess it depends on what we mean by 'opening up the hood.' Sticking to the analogy I would take that to mean cracking open the physical case. Most of the 'black boxes' in a cars engine are also protected with various DRM methods to make it illegal for others to reverse-engineer them (therefore making it so that mechanics have to pay the manufacturer for access to the devices). I would associate those with the actual chips inside of the iPad/iPhone as well as the OS.
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.