Cabal's system_default_prefix

[Sven Panne]

I've just seen

    system_default_prefix :: PackageDescription -> String
    #ifdef mingw32_TARGET_OS
    system_default_prefix PackageDescription{package=pkg} =
      "C:\\Program Files\\" ++ pkgName pkg
    #else
    system_default_prefix _ =
      "/usr/local"
    #endif

in Cabal's sources. What is the exact purpose and why is it "/usr/local" on
*nix and not "/usr"? The latter seems to be more natural to me for adequately
packaged SW. I must admit that I recently didn't have time to follow Cabal's
fast development, so it is quite possible that I've missed some details...

Cheers,
    S.