Pat K added the comment:
Thank you for the explanation. I understand this is intentional. However
user without such knowledge of inheritable permissions might want to
default the installation directory to the old one (C:\PythonXX) and
could easily run into this issue without knowing. IMHO extra
New submission from Pat K :
This seems to affect different versions of Python Windows installer. The
problem is when Python is installed for all users (requires elevation) its
binaries and DLLs are shipped with writable permission for "Authenticated
Users":
PS C:\Python36> icac
