There's nothing disorderly or actively harmful about that comment. It just adds little value compared to the search space of all possible comments. So it seems semantically more sound to simply withhold upvotes from it, rather than punishing it with downvotes.
Let's say a comment on one site gets 100 upvotes and no downvotes. On another site, another comment gets 10 upvotes and no downvotes. Was the first more popular, or just more trafficked?
You need the ratio of up-to-down to determine a comment's true usefulness.
You can't compare upvote numbers (or even upvote/downvote ratios) meaningfully between sites, because their communities aren't the same. Your proposed scenario tells us nothing beyond pure numbers -- 100 people voted on one, 10 on the other. You can't say anything comparative. You certainly can't speak to strength of intent.
