Is that really true? 

I just made the following:

#!/usr/bin/env python
# -*- coding: utf-8 -*-

import os
import sys

try:
    from setuptools import setup
except ImportError:
    from distutils.core import setup


required = ['httplib2>=0.4']
packages = [
]

setup(
    name='test',
    version='0.1',
    description='',
    long_description='',
    packages=packages,
    install_requires=required,
    license='',
    classifiers=(
        'Development Status :: 5 - Production/Stable',
        'Intended Audience :: Developers',
        'Natural Language :: English',
        'License :: OSI Approved :: ISC License (ISCL)',
        'Programming Language :: Python',
        'Programming Language :: Python :: 2.6',
        'Programming Language :: Python :: 2.7',
        'Programming Language :: Python :: 3',
        'Programming Language :: Python :: 3.0',
        'Programming Language :: Python :: 3.1',
    ),
)

Then I ran the following:

$ sudo yum install  python-httplib2
Loaded plugins: langpacks, presto, refresh-packagekit
Resolving Dependencies
--> Running transaction check
---> Package python-httplib2.noarch 0:0.6.0-5.fc15 will be installed
--> Finished Dependency Resolution

Dependencies Resolved

=========================================================================================================================================================
 Package                                  Arch                            Version                                  Repository                       Size
=========================================================================================================================================================
Installing:
 python-httplib2                          noarch                          0.6.0-5.fc15                             fedora                           35 k

Transaction Summary
=========================================================================================================================================================
Install  1 Package

Total download size: 35 k
Installed size: 35 k
Is this ok [y/N]: y
Downloading Packages:
python-httplib2-0.6.0-5.fc15.noarch.rpm                                                                                           |  35 kB     00:00     
Running Transaction Check
Running Transaction Test
Transaction Test Succeeded
Running Transaction
  Installing : python-httplib2-0.6.0-5.fc15.noarch                                                                                                   1/1 
  Verifying  : python-httplib2-0.6.0-5.fc15.noarch                                                                                                   1/1 

Installed:
  python-httplib2.noarch 0:0.6.0-5.fc15                                                                                                                  

Complete!
[josh@desktop test]$ sudo python setup.py  develop
running develop
running egg_info
writing requirements to test.egg-info/requires.txt
writing test.egg-info/PKG-INFO
writing top-level names to test.egg-info/top_level.txt
writing dependency_links to test.egg-info/dependency_links.txt
reading manifest file 'test.egg-info/SOURCES.txt'
writing manifest file 'test.egg-info/SOURCES.txt'
running build_ext
Creating /usr/lib/python2.7/site-packages/test.egg-link (link to .)
test 0.1 is already the active version in easy-install.pth

Installed /home/josh/b/test
Processing dependencies for test==0.1
Searching for httplib2==0.6.0
Best match: httplib2 0.6.0
Adding httplib2 0.6.0 to easy-install.pth file

Using /usr/lib/python2.7/site-packages
Finished processing dependencies for test==0.1

Notice how no downloads occur (it uses the one from the system).
Then if I remove it:

$ sudo python setup.py  develop
running develop
running egg_info
writing requirements to test.egg-info/requires.txt
writing test.egg-info/PKG-INFO
writing top-level names to test.egg-info/top_level.txt
writing dependency_links to test.egg-info/dependency_links.txt
reading manifest file 'test.egg-info/SOURCES.txt'
writing manifest file 'test.egg-info/SOURCES.txt'
running build_ext
Creating /usr/lib/python2.7/site-packages/test.egg-link (link to .)
test 0.1 is already the active version in easy-install.pth

Installed /home/josh/b/test
Processing dependencies for test==0.1
Searching for httplib2>=0.4
Reading http://pypi.python.org/simple/httplib2/
Reading http://code.google.com/p/httplib2/
Best match: httplib2 0.7.4
Downloading http://pypi.python.org/packages/source/h/httplib2/httplib2-0.7.4.zip#md5=37000b45eb3555c8b98faef0f400ddcf
Processing httplib2-0.7.4.zip
Running httplib2-0.7.4/setup.py -q bdist_egg --dist-dir /tmp/easy_install-FIpjgz/httplib2-0.7.4/egg-dist-tmp-GK5qyU
zip_safe flag not set; analyzing archive contents...
httplib2.__init__: module references __file__
Adding httplib2 0.7.4 to easy-install.pth file

So it will use the base os version if it exists, i guess the question is why isn't fedora using the base os version if its installed, or is something in glance making that not happen? Or is it broke in fedora...