Python requires a GIL. But Jython & IronPython don't. Why?

Why is it that you can run Jython 开发者_开发问答and IronPython without the need for a GIL but Python (CPython) requires a GIL?

Parts of the Interpreter aren't threadsafe, though mostly because making them all threadsafe by massive lock usage would slow single-threaded extremely (source). This seems to be related to the CPython garbage collector using reference counting (the JVM and CLR don't, and therefore don't need to lock/release a reference count every time). But even if someone thought of an acceptable solution and implemented it, third party libraries would still have the same problems.

Note that extensions written in C can in fact get rid of the GIL: http://docs.python.org/c-api/init.html#thread-state-and-the-global-interpreter-lock

My guess, because the C libraries that CPython is built upon aren't thread-safe. Whereas Jython and IronPython are built against the Java and .Net respectively.