No, there's a difference between outright spamming and the focus changing. There is not much spamming occuring, and most of what is just simply spam is removed. What happens is that the focus of discussion is changed through a natural transition. It's the same way conversations happen between people. Have you ever told your girlfriend to stay on topic?

If people are just spamming to be assholes then those posts will be removed. But if the majority of posts are just on a related topic, then removing that majority of posts is ridiculous because that IS the discussion.