[Haskell-cafe] Why do "unsafe" foreign calls block other threads?

Hey everyone, Could someone explain to me the logic behind having "unsafe" calls block other threads from executing? It seems to me that if anything it would make more sense for "safe" calls to block other threads since the call can call back into the Haskell runtime, as opposed to "unsafe" calls which (by assertion) will never call back into Haskell and therefore should be safer to run in parallel with other threads. What am I missing here? Cheers, Greg