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because that user having public privacy for the profle.
each user can set they own profile privacy and admin can site default privacy and reset all users privacy.
please goto jomsocial backend > configrations > privacy, set profile privacy to Members, click save button. and you can reset all users profile privacy by click "reset button"
yes that by design. do you want all jomsocial pages always private?
you need to set jomsocial at main menu item as registered access level, and create new homepage by using joomla article.