This part of the etiquette is obviously not uniformly respected. HN is still better at this than sites like reddit, but something tells me it's against human nature to upvote things we don't agree with - no matter how relevant they are.
In this case you don't have to agree with the statement, only agree that it's unpopular. Maybe they're disagreeing with the facts in the statement's premise? Then they should say so, I think.