Why prevent pasting of passwords?

I've been to more than a few sites now where if you paste the password they will not log you in. However if you type the exact same password, it's fine. Now I have a LOT of accounts and use a password vault program to store them. It's convenient to paste, especially as they are long secure passwords and by using the copy in the vault, someone looking over my s开发者_开发知识库houlder can never see the password.

So, my question is -- What are the reasons for preventing password pasting to a website? Does this actually increase security?

(Please don't argue the use of the password vault. At 200+ passwords, needing a different password for each, it's the most secure method I have)

Honestly, there is zero real value in it. Anyone can turn off javascript, which is the only real way to do this.

It's annoying as hell and just one of those things people do because they heard from a friend of their cousin's bartender that "secure sites do this".

My guess would be its to prevent somebody from sitting down at your computer right after you leave, and just pasting in your password to login. I doubt most people remember to clear out the clipboard after copying the password.

Obviously, they could just paste it into notepad and enter it, but by not allowing you to paste it, it makes it less likely you'll copy it.