Personally, I know of two cases from independent fields, where co-authorship was enforced. In each case, a junior scientist sent his original research article to an editor in the hopes of being published. After the review process one reviewer supported the application (suggesting minor improvements), the other completely rejecting it. Soon after, the junior scientist got a message from one of the big fishes of the field (presumably one of the anonymous reviewers), that the publication of the article will only happen, if he is included as an author on the paper. This would happen regardless to the journal the original researcher will try to submit to. To cite from one case in which the reviewer wrote: “You won't publish that without my name on it.”
There is no question about this being absolute malpractice. However, it obviously does happen. Despite only having a young career, I already know of two cases. So for me, the questions is how common is that? Is it one in ten? Or one in a hundred? How many case do YOU know?
If you are afraid of a backlash yourself, get in contact with me personally. I will treat your message with confidentiality.