The fundamental problem is that the easiest non-fluffy response to something is to object. If you read an article and agree but don't have anything personal to add, posting "This is correct for the reasons stated" is a waste of time, so you probably don't (and should't). But if you read something and disagree, you can simply write "This is not correct because X and Y" and you've added something (though not something very worthwhile most of the time, as X and Y tend to be knee-jerk emotional reactions).