This has been there from the very beginning (as it was a part of GreaseMonkey/UserScripts), but it's not part of the permission system. This is the developer saying "only enable the extension on these pages." So the system to enforce this is in place, but it's not treated exposed to the end-user.

IIRC (using Firefox nowadays) it did tell the user what sites it was allowed on, although it did this through wildcards (e.g. "http://news.ycombinator.com/*") instead of regexes.