I'm not really seeing the issue here. So the user agent is allowed (via Javascript) to modify the content the user copied on its way to the clipboard, but they'

I'm not really seeing the issue here. So the user agent is allowed (via Javascript) to modify the content the user copied on its way to the clipboard, but they're not allowed to read the clipboard without user consent, right? It's the latter that's scary (allows untrusted client code to see, for example, maybe my bank account number or a password if I used the system clipboard to move it around). I've seen enough legitimate use cases of fixing the browser's mistaken assumptions of how to translate HTML to plaintext to feel that the benefits to being able to edit the clipboard output offset the cost of putting up with a few "Want to read more? [mysite]" edits.