The point is mostly to make sure people are feeling as part of the community by the time they get downvote privilege, and downvote based on what contributes and what doesn’t.
> And why the people who can get to decide who can’t.
How does someone with downvote privileges get to decide who can't downvote? Simply because a downvote removes a unit of karma from the downvotee's score?
I think the point is to force people to lurk around a while before they can downvote anything.