Thats the way it should work, and is part of what makes HN different to say Reddit. In reality, its becoming increasingly common for people to downvote something simply because they disagree with it.
Edit: Would whoever downvoted this care to explain why? I'm genuinely interested.
It would be nice if Upvote meant "This is good quality (whether I agree with it or not" and downvote meant "this is low quality, and should not be here, but is not spam, nor flagable". But since PG has said (something like) downvote can be simple disagreement you're going to meet people who use it for exactly that.
Also, reddit is trying to move away from "downvote to disagree" - they have a lot of hover text saying this.
However that norm changed when people realized that was a bad idea.
The UI didn't change with the norm change.
At one point the norm was you could downvote to disagree if the comment had a score greater than 1. But now that comment scores are no longer displayed, that norm changed too.
Now the downarrow basically means spam/stupid/useless, etc. Actually for spam you also need to click on flag.
The reasons for clicking may not be symmetrical, but the actions when clicking are symmetrical. The points go down with the down arrow and the points go up with the up arrow. This is symmetry even if the reason (agree, disagree, quality, spam, etc.) for the increase and decline of the points is asymmetrical.