If I unlike an item it doesn't get removed from the news feed. While I understand it's a cache (I believe so anyway?), shouldn't it remove it anyway as it is no longer relavent (like accidental liking of content).
Otherwise, you could end up with this quite easily:
User caches are never rewritten, but the master data from which the caches are originally built is occasionally rewritten.
You can think of it like this: if someone has already read the news, there is no point in going back and removing that story from their copy of the newspaper, but you can remove it from subsequent publications.