There's an option to disable this here:
WebHost Manager »Server Configuration »Tweak Settings, Domains tab:
Restrict document roots to public_html [?]
Enabling this setting restricts Addon and Sub Domain document root directories to the same document root as an account’s primary domain (~/public_html)
You cannot enable or disable it from within your cPanel Account. Your Hosting Provider can, or not. That's up to them. If you feel you need, it, you'll need to ask them to enable it.