"Back" functionality is neither prevented nor treated as "Close" resulting in the page receiving this event. Although it can be handled at this level, but would be straightforward to handle it at the dialog level.

Please add a parameter that allows configuring how this should behave.

Suggestion:

public enum BackNavigationEventHandling 
{
   None,
   Prevent,
   CloseDialog,
   Callback // virtual method to handle the received LocationChangingContext 
}

Please note that except from the Callback scenario LocationChangingContext.IsNavigationIntercepted should not be prevented, but always result in the proper destruction of the dialog.