That's purely a library oversight which has become mired in backwards compatibility (don't track any of that onto my carpet :P).

It would be trivial to specify that all Monads are also Functors by changing the definition of the Monad class a little bit:

    class Functor m => Monad m where

It's a library/compatibility problem, and it's (finally) being addressed now (will take years until the change takes place, need deprecation warnings for a loong time).