As reported by Jeff Atwood, in Optimizing For Pearls, Not Sand:
We believe asking questions on our site is a privilege, not a right. If, after a few fair attempts, you haven’t been able to prove that your contributions to a particular Stack Exchange make it at least … not-worse … then we reserve the right to refuse your questions. If we don’t do our part to cull the bad questions, then we risk alienating the true experts who provide what really matters: the answers!
The main effect of down-voting a question is not reducing the reputation to a user, but making less prominent the question being asked. The effect of reducing the reputation of a user is secondary, and it doesn't reduce significantly the reputation of a user; for example, a user with a reputation of 1000 should get 250 down-votes before to see his reputation reduced to 500. Consider also that a single up-vote to a question has almost the same effect on the reputation of 2 down-votes.
Jeff then continue saying:
Continuing in that same vein, we have two more changes to formally announce today:
- We now limit users (and IP addresses) to a maximum of 6 questions per day and 50 questions per month.
- Downvotes on questions no longer cost the casting user 1 reputation, so they are effectively “free”.
The reason this has be done is that the number of bad questions increased, and they weren't sufficiently down-voted, or Jeff would not have taken those decisions.
Allowing to down-vote a closed question is not a flaw; a closed question is not a locked question.