He could, but it's unlikely he will close the site over improper downvoting behavior (which is rampant, but unimportant). And what I think people here should (and do) respect is the general spirit of civility; we respect Paul Graham's wishes because he asked for civility, not vice-versa. If Paul Graham asked us to be pointlessly cruel, we would not respect that (or we would be congregating elsewhere).

Therefore, what matters most in practice is the mass opinion of users of the site - what do we think is most useful and respectful in daily usage (following Paul Graham's lead to be respectful because we can all agree that this is a good idea).