Goes back further, even the C64/C20 was hard to squeeze the whole ram out of unless you wanted to scratch build everything. Even a Timex 1000/zx81 didn't get full use of the 2/1k ram. I'm thinking this has been standard literally forever (I'm assuming no one argued over usable space v total space pre-transistor).

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.