Why does the "confirm password" field exist anyway? It exists to remove the risk of input error. They want to avoid you locking into a mistyped password and not being able to recover. To this end, it makes some sense to prevent copy/paste, as a user may simply copy their mistyped password and paste it into the confirmation field. Especially risky if the input fields are obfuscated with placeholder characters (*).
Not to argue that it's the right answer, it certainly makes more sense than a heavy-handed enforcement of character limits.