There are two separate issues here, perhaps that is what is creating the confusion.
What the default settings during forum creation are for;
Enable active topics:
If set to yes active topics in selected subforums will be displayed under this category.
Display active topics:
If set to yes topics made to this forum will be shown in the active topics list.
And whether the two should have the same default setting or different default settings. In my world, the entire reason you choose a default feature setting for users is because you have reason to believe that default is the most desired setting by the majority of your potential user base, thus you preselect it for them. Do you agree with that premise? If so, good because here is where it gets complicated... the settings related to each other. They are related to each other because you provide two different "views" of what you define as 'Active topics list' and you do not provide any differentiation between them (to the user) regardless of the UI path taken to view them.
From the users perspective the 'Active topics' list can be displayed via a search:
or viewed from http://.../viewforum.php?f=1 <-where f(1) is a category forum.
From the users perspective the phrase 'Active topics list' can mean either of those two results, or both. The evidence of that exists in another bug ticket where you yourself referred to 'Active topics list' as (viewforum f=$category_forum_id) -> http://www.phpbb.com/bugs/phpbb3/57255#post223845
It is when you define 'Active topics list' as the viewforum page that the relationship between the two settings come into play. Active topics must be enabled for both the category forum and the content forum in order for the 'Active topics list' to display. When Meik changed the default setting for new forum creation he improved the usability of the application, you see very few support issues any more where the user is asking why they don't have an 'Active topics list' when they have 'Display active topics:' set to Yes. That is because both forum types are set the same and you no longer have to go in and change it for category type forums.