Why? "Haskell" also works on hugs,yhc,uhc,... http://www.haskell.org/haskellwiki/Implementations
Most implementations are BSD/GPL Licensed. Haskell itself (thinking of Haskell Prime, Haskell98) is a research/community driven project and, as such, open/transparent.
For GHC there is even an intermediate format allowing you to run your code on stuff like llvm.
the original message was talking of specific "stacks", which in my understanding includes an abstract language and a concrete implementation (I'd say also some accompanying development tools: if not an ide, at least a repl, debugger, build system).
"haskell" as such would not qualify, while ocaml, scala and ghc would.