JS_Harris - 7:10 pm on Jul 28, 2013 (gmt 0)
Actually I think they get that first comment on your site pointing to another site and that site points to their main site. This way, if your page gets penalized, they break the chain and protect their main site. You get all of the ranking problems and the chain no longer reaches their main site so theoretically they don't lose rank.
It takes a lot of energy to be lazy! Thankfully Matt Cutts suggested that these 'tiered' linking methods will meet their demise later this summer in an update he expects all SEO's to talk about.
I wish I could see the next generation of link evaluation technology in action at Google, it would be quite something to see. SEO's used to be able to know what a 'good' link was when it was based on two pages but now that links are evaluated in a systemic manner it becomes virtually impossible for SEO's to get 'the big picture'.