I think it’s because the shared category picker has one set of behaviors, based on the topic you are in.
Even if we are suppressing subcategories, though:
- why would “reply as linked topic” ever be limited to only the current category. That feels like a bug? At minimum other top-level categories should be shown to choose from?
And beyond that
- reply as linked topic is kind of a special case and should probably show the complete list of categories, including subcategories, but always default to the current category unless there are permissions issues preventing you from posting in the current category..