I have had a look into it and it looks like any category without subcategories does not link. I believe this might be because they were trying to implement a way to expand out the category to show the subcategories. I am unsure if this functionality is working. However I was able to get what you requested working with the following code:
Starting on line 60 in qa-page-categories.php I had the following code:
if (!$category['childcount'])
unset($navigation[$key]['url']);
I changed it to this:
if (!$category['childcount'])
//unset($navigation[$key]['url']);
$navigation[$key]['url']=qa_path_html('categories/'.qa_category_path_request($categories, $category['categoryid']));
By commenting out the unset line and adding the last line, all of my categories became links,