How did Apple set the precedent for that? If you're referring to iOS devices, then they definitely did not set the precedent there. PC manufacturers have advertised total space, not free space, for ages.
I was referring to iP(ho/o/a)(d/ne)s at 2/4/8/16/32GB, losing a decent percentage, without this type of outcry. Sealed devices, marketed on HD size.
That doesn't mean it's right of course - space post-os would be a great spec to have. Also a motivation for more people to question why. A lot of the overhead (CPU, ram and long term storage) on a regular computer or phone is there for good reasons, but I would venture to say the majority of "no-load load" is questionable at best.
I wiped Xandros, installed what was called Eee Ubuntu (now Easy Peasy) and never looked back (though I since got a bigger SSD, maxed it out on RAM, and happily run Bodhi Linux)--but I can only imagine the wrath of someone who bought one and didn't know that was an option, especially one bought as a gift. I had to wonder whether it was set up to fail.
But there is one huge difference you have overlooked - with your 486 you could install any operating system you wanted. You could even delete bits of DOS you didn't use. With the Apple/Microsoft tablets you can only run the operating system they provide (cryptographically/hardware enforced) and you cannot delete any of the OS.
With that 20MB 486 you could choose what every single byte is used for - and that is true of PCs today. But not for the locked down hardware.