I think that http://www.haskell.org/pipermail/haskell-cafe/2008-April/041461.html may be relevant. It's a design decision.

Thanks for the link. I've read through the thread, but rather than try to figure out if it's the same issue and whether it's a design decision or a historical accident, I've decided to create a ticket (#2885) and let GHC developers decide if it's valid or not.