content: There are a ton of things websites can do that are unsafe. Allowing people to put things onto your clipboard can have legitimate uses. Say instead of having to highlight somebody's post and control+c / other method of copying you just click a "Copy" button. I would personally agree that browsers should be defaultly safer and give you a warning that somebody wants to mess with your clipboard. Especially since a "safe" website could have unsafe ads trying to mess with the clipboard (or some other feature).