Not sure of the scope of this behaviour...
Have implemented "SubCategories" based on the code-example "PropertySubCategory".
Just deleted one category and all pages with that category had the category replaced by it's non-selectable parent-category.
ID ParentID Category
8 X <--- pages got this 9 8 y><--- deleted this 10 8 z>--->--->
Que?
ID ParentID Category 8 X <--- pages got this 9 8 y><--- deleted this 10 8 z>--->--->
Que?