Why does the files directory ignore system wide umask?
Yes, if Drupal has permission to write to your doc root then it will certainly create the files folder on your behalf during installation, and attempt to set sensible permissions on it (775 seems perfectly reasonable to me considering the web server itself is creating the folder). This all happens in the /includes/install.inc file.

If your server is secured correctly, though, this should not be able to happen. The web server user should not be able to write to the doc root, only to the public/private filesystems. So you would create the files folder manually, and whatever permissions you decide to grant become part of that process.

See Securing file permissions and ownership for a full set of instructions on properly securing a Drupal site.