The keyboard remapping in the Registry Editor is done at the kernel level, so of course it is a system-wide setting. There's a new PowerToy for Windows that does key remapping at the "window manager" level and does not need administrator access unless you want to also remap that key in administrator applications.

Most of the other "normal" Setting/Control Panel changes that need UAC prompts for certain classes of users have similar explanations. What Windows considers a system-level or multi-user setting certainly differs than *nix, but it doesn't necessarily mean there isn't a good reason why it differs.

Some of that is that Windows (NT branch, XP+) is a multi-user operating system most widely deployed by people that assume it is a single-user operating system and exacerbated by confusion where Windows mostly pretends to be compatible with a single-user operating system legacy (3.x/95-ME) right up until it cannot anymore and sometimes the multi-user-ness comes out to bite.

True, but that's still not really what I'd call random or arbitrary.

registry settings are things that should be user settings

User settings are most of the time registry settings since that thing is used where other OS use plain files :) But I know what you mean, some of the choices which were once made are strange and because of the sometimes annoying "let's keep things backwards compatible forever" principle it's also not likely to change soon. Then again, pretty sure for remapping there are workarounds in the form of creating your own keyboard layout for which MS has tools. Now if changing a keyboard layout still requires a UAC prompt (I really hope it doesn't) you're still nowhere.