As a follow up.. as we have a policy of always categorising a topic i thought we could secure the uncategorised category. Looks like that is not an option.
Perhaps the easiest change would be the addition of a forum setting that lets you nominate a specific category for Make Public that defaults to uncategorised. That way folks could set up a secure transition category where they have a chance to categorise the topic appropriately without fear of inadvertently making sensitive content public.