Why does Python keep a reference count on False and True?

I was looking at the source code to the hasattr built-in function and noticed a couple of lines that piqued my interest:

Py_INCREF(Py_False);
return Py_False;

...

Py_INCREF(Py_True);
return Py_True;

Aren't Py_False and Py_True global values? Just out of sheer curiosity, why is Python keeping a reference count for these variables?