if an answer is wrong , then we can vote it and the wrong answer goes down the list,but why decrementing the hard-earned reputation??
we need to bring a change to that policy.
I think that it has been addressed previously (I can't find a dupe), or maybe it is too obvious.
This restriction makes you take it more serious. If you don't like a post, even when it may be useful, you would likely downvote it just because.
IMO it is a perfect policy that doesn't need to be changed.
At the end of the day, as Eliah says:
if a post isn't useful to anyone, we certainly should downvote it!