Hi David,
You’re right that when you add a permission for someone to access your data, you are adding permission only for that user, not for all users.
The Public/Private setting determines whether users can find your share and view the attributes of your share, such as the display name of your share, the owner, description, keywords, uuid, etc. The Public/Private setting does not determine who can view/access the files and folders on your share. Permissions are used to grant read/write access to the files and folders on your share.
Shares are always set to Public so that people you’ve granted permissions can find your share in order to access data.
I can see how "Visible To: Public -- Shares are visible to all users” can be confusing. We’ll update that language to make the explanation of the setting clearer.
Brigitte
Globus Team