Date structure of tasks and subtasks

Hi!
    I had been working on the date constraints bug: https://bugs.launchpad.net/gtg/+bug/826916 and the question asked related to it https://answers.launchpad.net/gtg/+question/197755.
   I was confused at one point:
     we have a task structure
       A (some due date)
         -B (no due date).
  Now as discussed, my code did not update the due date of B when due date of A was updated. But, when we call get_due_date for B. It would return the due date of A as it returns the most urgent due date of it's parents. As a result, the due date of B will not be refreshed in the task browser after updation of due date of A.
   But, then when we update some attribute of B, Bs due date will be updated to that of A.

   So, my question: should get_due_date return the most urgent due date of all it's parents. If yes then why not update the due date of the child when due date of the parent is updated? Because, I feel it is a bit odd that get_due_date would return something else and actual due date would be something else.