The requiring a click part is the only relevant implementation detail for this functionality, in my opinion. Whether or not it requires a plugin shouldn't matter.
In fact, requiring a plugin for this functionality only serves to increase the attack surface for the browser as a platform.
Flash code can read clipboard from Flash/browser i.e. it can run in background and silently and continuously collect contents of clipboard? If so then it should be rightfully classified as spyware and purged from every computer.
I think Flash only allows SWF content to access the clipboard in response to a user click.
GitHub's "copy repo URL to clipboard" button uses Flash to write to the clipboard. Shumway has implemented Flash's clipboard APIs (which it can do because it can access Gecko XPCOM APIs as a Firefox extension), but they are currently disabled because Shumway hasn't implemented the "in response to user click" bit yet.
