BTSync is only usable by the Master account, we never laid claim that this was available to all users. Remember please that multi-user availability is only a courtesy and does not include all the bells and whistles of the master account.
You will need to take it upon yourself to make these adjustments within the BTSync application. Pretty sure you can add in the need for login to the application (from within the applications settings itself) You may additionally setup rules in your vhost for BTSync to RequireUser and this will require a username/password to be included.