That's because sometimes vested interests (like say the military or big corporations) will actually use sophisticated software to flood web forums with fake conversations and comments on topics they want to sway opinion on.
The practice is called "astroturfing." Apparently its becoming more and more common.
Basically, they create fake "grassroots movements" that pour their "opinions" onto a web forum about, oh I don't know, let's say the evils of big tobacco.
These virtual movements are created by those companies to make it appear that great numbers of the public actually approve of what they're doing.
So there you are on a web forum and it seems like 500 dudes are all, "Hey! What's your problem? Black Lung Tobacco Inc. is a great company! They care about the environment, and they love puppies too, so what's your damage, hippy?!"
And then maybe you start to think, "Well... Am I being too hard on Black Lung Tobacco Inc? I mean hell, if they love puppies...."
But guess what? Those 500 dudes don't exist. THAT is extremely dirty pool.