According to the documentation of subprocess, as soon as you specify the env parameter, it does not use the os.environ. Since I want to add to the existing env, I copy it first, then add my own.
From http://docs.python.org/2/library/subprocess.html
"If env is not None, it must be a mapping that defines the environment variables for the new process; these are used instead of inheriting the current process’ environment, which is the default behavior."