Because sometimes when a process is implicitly started by some 3rd party library (i.e. COM via pythonwin here) the "old", unchanged environment is retained as the process itself doesn't care if os.environ was changed or not, the original environment cannot be modified.

This solution would give a lot of flexibility in that regard.

I think pythonwin is actually a special case as the problem here lies somewhere in the dispatch(.) call. The process creation should be handled somewhere within the COM interface, pywin does nothing there. 

But this solution effectively circumvents that issue.