iUnix security is basically built on top of restricted access. If you own the machine and the boot disk, you can "break" it anytime. Linux is the same. iPhone is yours after you have bought it, surely you don't want apple or AT&T control that phone. Just like you buy a dell computer, you don't want dell owns that admin account.

There are two different kinds of iPhone unlock: unlock the SimLock which allows you to use different carriers GSM system. SimLock has been widely used, and attacked. Perhaps some hackers have broken the GSM SimLock on iPhone. The second one is the media player which iPhone requires specified activator to be activated. Someone has broken that also.

Bootloader is typically used in embedded system development, and which is necessary during development and deploy phase. Of course it is easier to hack iPhone with bootloader, since you can bypass many security locks. It is possible to make it so almost impossible to break, however, that requires special hardware and software. However, it is almost impossible to get export licenses for those things.