I think I know what the problem is - there's a difference in the topics list available in "News/Activism", which is the one you and I are familiar with, and the topics list here in "General Interest". Try it out - open two browser windows, one for News and one for Chat, hit "post" for both of them, and bring up the topic lists side-by-side. For whatever reason, "Philosophy" is not available as a topic here in General Interest - guess it's not something that freepers are "generally interested" in ;)
Anyway, that's why I missed it - it wasn't there in the first place...