Following the documentation,
os.path.normcase(path)
Normalize the case of a pathname. On Unix and Mac OS X, this returns the path unchanged; on case-insensitive filesystems, it converts the path to lowercase. On Windows, it also converts forward slashes to backward slashes.

on Mac OS X,
>>> import os
>>> os.name
'posix'

Checking through, Lib/posixpath.py,

# Normalize the case of a pathname.  Trivial in Posix, string.lower on Mac.
# On MS-DOS this may also turn slashes into backslashes; however, other
# normalizations (such as optimizing '../' away) are not allowed                
# (another function should be defined to do that).

def normcase(s):
    """Normalize case of pathname.  Has no effect under Posix"""
    # TODO: on Mac OS X, this should really return s.lower().
    return s

Why on Mac OS X, it should return s.lower() ?
Also to raise Error for None case we can probably change normcase() in Lib/posixpath.py as,

def normcase(s):
    """Normalize case of pathname.  Has no effect under Posix"""
    if s is None:  
        raise AttributeError
    return s