It comes and goes in cycles, and its usually removed in minutes. What the spammers are doing is trying to get Google and other search engines to pick up on the link, and raise the ratings of the business they are promoting. Google scans CR frequently, so its a choice target.
SEO spamming is a big issue, spammers sell a business on raising their Search Engine ratings, charge them a monthly fee, and then spread SEO spam. Many businesses do not even realize that their good name is being tarnished, they see their search engine results rise, and then when Google imposes a ban on them or drops them 200 places, and they lose their money and reputation.
This even happens to big companies, JC Penney got caught a while back. If its too good to be true ...