When a user assigns a task on a document, and then leaves the enterprise, the task is deleted from the document when the user is deleted.

The task is actually a part of the document's lifecycle, not the lifecycle of the user. If the content is owned by the enterprise, the termination of the user should not cause the task to be deleted.

Our use case is this:
Internal auditor gathers content and places it in Box
Internal auditor assigns a task to the Finance Director and CFO to review the content
Finance Director and CFO approve the task.
Internal auditor then leaves the company
Internal auditor's account is deleted so enterprise does not incur cost associated with an account that no longer exists.
When external auditors come in to check that content has been reviewed and signed off against, the tasks were deleted from the documents.
Organization gets penalized for not reviewing content.

Preferred workflow:
Internal auditor leaves the company
Internal auditor's account is deleted
External auditor comes in to ensure content has been reviewed and signed off against
Content has the approval task still in it (although the "previous user" tag is listed as the originator of the task)

Again - tasks live with the DOCUMENT not with the USER.