Interesting cultural bias. Why are people not questioning unsubstantiated up-votes, but feel down-voters shouldn't be let of the hook without a full argumentation?
Upvote is already backed by the arguments the upvoted post contains. Downvote is not backed by those arguments. They are not symmetrical at all and there is no bias.
That is only true if the upvoted post contained an argumentation and not just a statement or an observation.