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

[Gregory Crosswhite]

 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