PHP iCalendar, the software used to display the Calendar, is a separate software program. It does not know that you've already logged in to Tasks Pro™.
In order to access the iCalendar that Tasks Pro™ creates, PHP iCalendar has to send your login information.
The iCalendar that Tasks Pro™ generates and that PHP iCalendar reads uses 401 authentication for security. This method is supported by most iCalendar applications to allow you to use the iCalendar with the software you choose.