I can't understand the reason behind it. By voting I participate on the site and in my opinion, downvoting should not be discouraged like that. There are already all these nice popups on the site ("consider leaving a comment", "you have x votes left today", "please come back in x hours to be allowed to vote again" etc...)
I will continue to downvote whenever appropriate and don't care about my reputation that much and I do consider the effects on morale and I consider wheter to leave feedback.
I'm just curious, though. Does the system detect a downvote without comment (possibly giving feedback for improvement)?