Acceptance of ghostwriting, euphemistically termed “editorial assistance,” may be so widespread that it is considered normal.
http://plosmedicine.org/article/info:doi/10.1371/journal.pmed.1000335#article1.body1.sec8.p3

This sentence seems to be confusing some quite different concepts. First, ghostwriting. This is the kind of thing described in the rest of the article, in which unacknowledged and hence invisible writers are involved in an article. I don't think any mainstream commentators consider this to be acceptable.

"Editorial assistance" is an unfortunate phrase, as it has often been used very loosely in the past. It may mean simply editing an article written by the named authors to improve grammar and the like, or it may mean that a professional writer actually drafted the whole article. In the EMWA guidelines, we specifically discouraged people from using the phrase "editorial assistance" in acknowledgements and encouraged a more precise description of the writer's role. It used to be a phrase commonly seen in acknowledgment sections, but seems to be becoming less so.

So, ghostwriting and editorial assistance are certainly not the same thing. Editorial assistance may be ghostwriting if it is not declared, but provided it is declared in a way that makes it clear what form the assistance took, and of course given that the authors still take responsibility for the content of the article, it is perfectly acceptable, and indeed desirable, given the benefits in reporting quality that professional medical writers can bring to publications (see references in my main comment on this article for evidence of those benefits).