Disagreeing with a post is not necessarily non-constructive. If I make a suggestion about a game, and get a bunch of disagrees, that's pretty constructive communication - the suggestion is clearly unpopular. Replies explaining why it's unpopular may be useful, but the disagrees aloe do communicate something useful. So too with more general discussion - if someone lays out a theory about game strategy, or complains about an upcoming change to a game, disagreeing with that post communicates something useful both to the poster and to everyone reading the post.
Moreover, disagreeing with a post is not passive aggression. I don't think it's particularly aggressive at all, but if it was it could only be construed as active aggression. Going up to someone and telling them you think they're wrong is by no means passive.