why cannot I edit admin permissions for the permission page in D8? All permission boxes for administrators are disabled "greyed out". 

I can do it via drush but not through the UI.
That's because administrator is an admin role, that means it has all the permissions all the time. Editing it via drush won't do anything.

If you want a role not have all permissions it must not be an admin role, but that means you need to manually add new permissions.