Wait, that doesn't sound quite right. The AER has dropped its section for shorter papers, but that doesn't mean it has adopted a policy of not publishing short papers.
Part of the problem with the shorter papers section was that there was never really a separate policy for short papers. Years ago, I had a paper come out in the short papers section, and someone pointed out to me that it wasn't short, in fact it was the second longest paper in the issue (or something like that). The editor at the time was Orley Ashenfelter, and so I emailed him and asked what had prompted him to accept it only as a short paper (since it wasn't in fact so short). His answer was something like "I just didn't think it was a very good paper..."