I don't know for sure, but I think it's installed under /nix thinking of multi-user Nix installs. With Nix any system user can install its own set of packages and on NixOS there are build users that install packages on behalf of normal users.
Yes. I'm unconfortable doing this as well. Not only with Nix, I really think you shouldn't do it, ever. On Ubuntu/Debian, at least, I'm installing the .deb package.