Looking for confirmation that there is no way to prevent someone from closing a work item that has open tasks assigned to it. In this specific case, Defect work item with open tasks. Anyone with Editor or higher access can close the Defect even if there are open tasks is that correct and if so why?
At this time we do not have a way of preventing users from closing a defect with open tasks.
We do, however, have "Exit Agreements" on our Team Board that could display that information. When teams use the team board to move artifacts across the board the exit agreements can be used to remind users of this step.
This link has detail about how teams can take full advantage of all the Team Board's features: Team Board
This link has detail about Exit Agreements: Exit Agreements
Also, users can add Task fields to the Team Board that would display the State of the Tasks. These fields can also be added to the Defect page.