1. Use webservice core_user_get_users and core_user_get_users_by_field to get the details of a user who has set in his/her profile 'allow only other course members to see my email address'

2. View the result. Even with full admin rights for the webservice user their will be no Email address in the result.

Now, change the profile of the user to 'allow everyone to see my email address'.

Repeat step 1 and you will get the email address in the result.

------

It could be that this is by design or it could be a bug. What happens here is if external systems try to maintain the user accounts in your Moodle system, you can not update the mail address of the user (this could be necessary with marriages, divorces or deaths of partners)

For maintaining user accounts this is a problem.