I think installing to USER_SITE by default is fine and preferable. But there was something wrong with the way it's implemented: if I specified --prefix $PREFIX, I got some files (python modules, etc) installed to $PREFIX/lib/python2.7/..., while some files (looks like all from data_files=[...] list) to $PREFIX/home/$USER/.local/...

This change makes --prefix work correctly, while still allowing user to install to USER_SITE by doing 'python setup.py install --user'.

There might be a way to make both work (--user by default and --prefix works as expected), but I'm not disutils expert and can't figure out how to do it without going too deep into distutils code.