Isn't that very anti-linux though, to have a directory owned by root but populated with subfolders owned by other users? /home is the only exception I can think of that does this.

Anti-linux I don't know, but it was not uncommon in unices to have home directories in /usr/home.

And there is no written or unwritten rule about that. In fact, /home is a subdirectory of / which is owned by root.

True, but /usr/home is no longer a common place to store home directories. It used to be, particularly in Bell Labs Unix. (Does FreeBSD still do this?)

The Linux Foundation’s File Hierarchy Standard puts user homes in /home, but it’s by no means mandatory.

/home being the *nix home folder directory isn’t written in stone, but plenty of software expects it. Of course you shouldn’t hard code things like that, but that has never stopped anyone from doing it. (Not that we should reward that with de facto standards necessarily.)

I understand the various reasons why a root file system hierarchy isn’t part of the Single UNIX Specification, but it might have been nice.

/tmp

And /run/user

also mail and cron

There's no other path that the user is guaranteed to have write permissions to (except maybe /tmp, I guess).