Murky language

Explicating the difference between "adult" and "aimed at adult aged demographics" certainly gets murky when talking about manga (as rather timely discussion has proved) but a rather puzzling addition to a few Amazon listings leaves me confused not so much about the language as to the "why?" factor.
Lo and behold, a trip over to Amazon to look up some ISBN numbers turns up a new (and I'm sure it's new, because I certainly would have noticed when looking up these books before for ISBNs and street dates when I reviewed them a month or so ago) tag add on to certain books:
"Adult."

Case in point, on "After School Nightmare"

First and foremost, it makes the series sound a bit seedy beyond words (re: what does adult mean to you?) and secondly does it a bit of a disservice, particularly since the books, while occasionally violent and definitely a psychological trip session are nonetheless pretty benign in comparison to what we would typically define as adult fare. (Gantz, anyone?)

So my question here is, how did this get here? Was this tag Amazonian in nature, or did Go!Comi take it upon themselves to do so? (Who else but the manga blogosphere could even begin to tackle this question...)