How short is too short?

I got a rejection letter recently, from a journal I will not name here, which told me that the article I had sent them, at 6000 words, was not long enough for their journal, where standard articles ran 9000.  This seems to me somewhat of a silly reason to say no to a piece.  I would like to think that 6000 words is plenty for making the kind of point that an article is intended to get across.