[Cthulhu_]

As a naive non-security expert, I'd consider it a risk to put passwords in the clipboard - what if there's something that can read your clipboard? JS on another site for example? What if you accidentally paste it on a 3rd party site?

That's copying it though. I know a decent password manager will clear the clipboard after an X amount of time too, mitigating the risk somewhat. But that's copying, not pasting in a field.

[foo101]

How does a site disabling "paste from clipboard" protect my password that I have already copied to clipboard?

Note that I would copy the password first and then I would realise that the site is not allowing me to paste it.

[virtualized]

If a browser allows websites to read the clipboard, that's a security bug in that browser.