I found the following article that introduced me to the term “SEO poisoning.”
I was considering this Cross-Site Scripting, but the code wasn’t malicious is was just leading back to another site selling their services. The term SEO poisoning makes more sense. The goal of this poisoning is to increase the rankings of the company performing the action by embedding links into a legitimate site and linking back to their site. Link backs are part of the matrix Google uses to rank your site.
The links are often displayed off screen so webmasters are usually unaware of the poisoning even happening. Fortunately some of the writers of the plugins are aware of this technique are incorporating tools into their plugins.