This has also proven to be correct in Amazon's case, whose site's possibility to create user and seller profiles has allowed spammers to advertise various adult sites such as adultmediareviews(dot)com and redspacetube(dot)com (some of which contain keywords that suggest illegal content) and fake pharmaceuticals:
A simple search of the domains in question and taglines employed in the profiles shows that there are many that serve only to promote these sites:
The redspacetube(dot)com domain has also been spotted in the past redirecting users to pages where fake AV solutions are offered. And while one could say that people searching for illegal adult (or not so adult) videos deserves to have their computer infected, I doubt that the spammers really care who the victim is.
The Zscaler researcher who has been looking into the matter has notified Amazon of the abuse of their site's functionality, and it will hopefully spur the company into doing something about it.