When you respond to his post it causes the google

When you respond to his post it causes the google ranking for the post to increase and therefore people find it when hey search for certain words and terms. This is why they post it. They want people to respond to it and it's relevance to google (and other search engines) increases. Even if the post gets deleted, it stays in the google cache and then all people see is the cached version of the post and they start believing the negativity and misinformation.