There had been a lot of discussions over it. For example, one of my other addons FoxyTab, doesn’t have any content script at all. However, it can copy Tab title and in order to copy anything to clipboard, WebExtensions (soon to be called Browser Extensions … Moving the WebExtensions docs), addon has to inject a code into the content which is not possible on above sites, and there isn’t any Clipboard API at the moment.
https://bugzilla.mozilla.org/show_bug.cgi?id=1344410
https://bugzilla.mozilla.org/show_bug.cgi?id=1197451