I believe the motivation behind it is to put emphasis on up-voting or not voting at all. This way, down votes will carry more weight and it will also prevent users from abusing the system by down-voting excessively:
The trick here is that downvotes are mostly informational. The cost of a downvote to the users’ reputation (or karma in Slashdot/Reddit parlance) is quite low. It would take a whopping 5 downvotes to equal the effect of a single upvote. And, on top of that, downvotes cost you a tiny bit of reputation. The net effect is that you have to feel very strongly about something to downvote it. Downvotes are serious business, and not to be cast lightly. We designed our system around that maxim.
-- The Value of Downvoting
Nevertheless that should not stop you from down voting something which is obviously not correct or not an answer. But if you value your score and think something should be done about a certain answer, you can also flag it for moderator attention, or just leave a comment and not vote on it at all.
I just checked and you did flag it, which I responded on by converting that answer into a comment (which it obviously was indeed). Not sure if you have gotten your lost point back now though, there isn't much I can do about that unfortunately.