It raises support costs, and frustrates the vast majority of customers.
If you're not satisfied by those restrictions, use Android. (I do, because I'm not.)
If that is not enough, the unlocking process can be as complicated as needed to discourage careless unlocking. Most people, when asked, e.g., to convert a decimal to a hexadecimal or to engage their frontal lobes in some other way, stop being careless.
Remember that the only point I'm trying to make is that it is not strictly necessary for Apple to do their best to keep motivated technical users from escaping their "jail" to prevent kids and grandmothers from harming themselves.
And does anyone actually believe that protecting naive users is the only reason Apple makes it as hard as possible to jailbreak iOS?
I wish I had your circle of non-technical folks. I've been called (from other people's phones, no less) numerous times asking why data doesn't work (they deleted all the APNs from an Android phone), why their picture messages don't go through (they changed the MMSC URL to their homepage), why voicemails stopped (they set the voicemail service number to their own number), and several more. Changeable settings are like mountains; they're messed with because they are there.
