If I had to guess he's shutting it down in large majority because of the tiresome spec work debate
It should not be confused with pro bono work, where the work is unpaid but the relationship between client and designer is transparent.
The practice is by no means limited to the design industry, but due to the nature of work involved, is where it is most rampant.
some of the arguments for and against can be found in the comments of this post on signal vs. noise by crowdspring http://www.37signals.com/svn/posts/1253-the-nospec-campaign-...