[Distutils] does easy_install.py need to guess the site-packages path?

As an example I'm looking at setuptools-0.6c11-py2.7.egg, can someone tell me why setuptools/command/east_install.py in get_site_dirs() does its own computation of where site-packages is? As of 2.7 there are already 4 separate places in the python where this path is computed. Maybe I'm missing something but I don't see why an egg would have an opinion on this matter. Regards, - James