Because the bigots will usually share these links encouraging their allies to reinforce them in the comment section. It would just take 10-20 individuals with nothing better to do to make it appear that most of the reaction to the article is negative. While you could say that dreamers and their supporters could do the exact same thing, dreamers have better things to do than spending time debating against close minded jerks. Right wing nuts are extremely vocal on immigration issues, and will post a negative response every chance they get.