I'll bite.
The iPad is by definition a more limited environment in that it IS locked down and heavily sandboxed. What you get from that is a hyperportable tool that pretty much Just Works All The Time without the need to worry about security or administration.
There are absolutely things you can't do there. For any advanced user, it's unlikely to provide a sufficient computing environment in and of itself -- but there are lots of folks for whom a nice iPad is entirely enough. My 80 year old mother only rarely touches her laptop nowadays (realistically, it exists for the version of Quicken she likes + the need to write embroidery patterns to flash drives for use with her INSANELY complicated sewing machine).
This is good.
But you trade some things away by setting the dials in that way (high portability & battery, high security) -- obviously, there's less direct user control, and less ability to customize, and outside environments like Pythonista I don't think there's any way to write code on it you could use on it.
But that's fine, because if the iPad doesn't meet your need as a primary, you probably have a regular computer on your desk, too. A Mac works just like computers have worked for 40 years -- you own the machine, down to the metal, and can do anything you want with it. This freedom comes at a cost (increased risk of crashes or malware infection), and may not represent the dominant mode of computing for regular humans going forward. That's fine. Not everybody needs that.
So the distinction is actually really simple: They're different products, positioned to address different user stories. Why shouldn't they have different approaches to lockdown/security/customization?
I mean, car makers have several models available, too, with often very very different capabilities and functions. That's also ok.