Setup.hs compiled w/ `-threaded` even though `cabal` built with `-DCABAL_NO_THREADED`

I was surprised to see the following isn't done when `CABAL_NO_THREADED` was set:

``` diff
diff --git a/cabal-install/Distribution/Client/SetupWrapper.hs b/cabal-install/Distribution/Client/SetupWrapper.hs
index 24a0464..91b56a6 100644
--- a/cabal-install/Distribution/Client/SetupWrapper.hs
+++ b/cabal-install/Distribution/Client/SetupWrapper.hs
@@ -548,7 +548,7 @@ externalSetupMethod verbosity options pkg bt mkargs = do
(program, extraOpts)
= case compilerFlavor compiler of
GHCJS -> (ghcjsProgram, ["-build-runner"])
- _ -> (ghcProgram, ["-threaded"])
+ _ -> (ghcProgram, [])
cabalDep = maybe [] (\ipkgid -> [(ipkgid, cabalPkgid)])
maybeCabalLibInstalledPkgId
```

is there any reason?