How do your user create the pages ?
I recall having a customer with the same issue and we solved it using the Search module that include a "Edit/Add page" option with a set category or default category (something close to this )
What is cool is that you can place it anywhere and so create an "input" page to be sure the inputted content get the right category.
Helas, our doc is not up to date on this and if you manage to make it work and can improve the doc that will be awesome ! (https://doc.tiki.org/module-search)