The question is WHY not what type!! You are a merciless beast. That's the second time for "O, Well". Next time I should give you a few more minutes to finish editing your post.
They are made threadsafe for the obvious reason that immutables should be copied when they become mutants.
Refcounts are made threadsafe because "immutables should be copied when they become mutants"? Sounds like a Tragedy by the Bee Gees.