It's (probably) because you see it as (1) an attempt to play on people's sympathy for the underdog and get upvotes simply for having been downvoted, and (2) an attempt that's likely to work. So (1) you want to discourage such rhetorical trickery and (2) you want to undo the damage it does.

In many other fora with karma systems, I think that's entirely reasonable and correct. LW is (in my judgement) less afflicted than average with exploitative whining about being downvoted; perhaps the response is less appropriate here.

(The other thing about waah-I-was-downvoted complaints is that they divert discussion onto what's usually an irrelevant side topic. Like what's happening now; oops.)