Well, like any social site, there is a sense of community. If you just dump your feeds, you are just self-promoting. Why even sign up for the service at that point if you are just going to spam it? I find it hard to believe that people could read their friendfeed river and not at least “like” one post. I am not saying that people have to comment a lot or even be that active. Just showing a little interest in some items on friendfeed qualifies as “activity”.

Think of it as if you are submitting stories to Digg/Mixx/Reddit. If you only submit stuff but don't comment, vote or anything then you are basically considered a spammer. That is generally accepted on those sites, what makes friendfeed any different?