Clearly, these businesses are not at all concerned about the quality of their work. And I suspect they haven't been keeping up with developments in their own industry. While spamming comment threads may still work as a way to lift rankings in the short to medium term the effect must certainly be waning. Nowadays, any URL associated with the practice risks being slapped pretty heavily by Google, or even completely removed from its index.
The fact that this practice is still so widespread is another reason that website owners should do as much of their own SEO as possible., in my opinion. Imagine pouring thousands of dollars into such a campaign thinking that it was completely legitimate, only to see your search engine traffic disappear overnight!
That's not to say that all blog commenting is bad. There is SEO value in leaving good, relevant and thoughtful comments because you do get backlinks -- but most importantly you are alerting other bloggers to your website's presence. If both your comments and your website content are of a good quality, then a few of them will probably link to one of your pages in time. Therefore the best way to approach commenting is to only ever write ones that you would leave regardless of whether you even had a URL to link back to or not.