There are 2 reasons:
1) apps are soft-deleted in zamboni : https://github.com/mozilla/zamboni/blob/master/apps/addons/models.py#L337
2) django usually handles the cascade itself instead of relying on the db to do it : https://docs.djangoproject.com/en/1.6/ref/models/querysets/#django.db.models.query.QuerySet.delete