But it's still a box sealed shut that someone else built. And, perhaps most devastating, it's designed to just work out of the box.
I'm drawing an analogy to the maker movement here. I don't want childrens' experience to be defined by others, or to live in the boxes invented by others, whenever I can avoid it. Left to their own devices, they invent their own boxes all the time. I would have never imagined that one of our trees would be a pretend locomotive, but there you go.
My first computer, a TRS-80 CoCo II, turned on to a BASIC prompt. Yes, you could run software others wrote. But you were almost inevitably learning concepts about the computer, too - how can you copy data from one disk to another when you only have one drive and your RAM is smaller than it, for instance? Sure, you can use something Tuxpaint on an iPad, but you wouldn't be using it on something you built.
Apple's ecosystem is the antithesis of the "I can do it myself" approach. (If you do jailbreak it, then I start to see some more value.)
Linux is the embodiment of the "I can do it myself" approach. I remember as a kid opening up a hex editor and modifying the boot sector of a floppy so that when you forgot it in the drive at boot time, you saw my name instead of the "Non system disk or disk error." But my boys will have source code, if they should ever want to use it. How cool is that?