All code referred to is from bltinmodule.c, Python 2.7.8:
https://github.com/python/cpython/blob/2.7/Python/bltinmodule.c

filter implements and optimization for PyBool_Type, making it equivalent to PyNone:

    # Line 303
    if (func == (PyObject *)&PyBool_Type || func == Py_None)

The specializations for tuples, byte strings and unicode don't have this:

    # Lines 2776, 2827, 2956, 2976
    if (func == Py_None)

This is a damper against recommending `filter(bool, ...)`.

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Python 3 of course does not have these specializations, so has no bug.