WordPress as well as BuddyPress has little hooks that when certain actions are accomplished it will run a function. So BuddyPress as hooked in the the post hook and posts an activity update about that blog post. You probably could do the opposite but the activity update is missing some content a post would require, like the title as the activity update is just content.
Another issue would be to separate out what is considered a blog post and what is not on posting. I personally would not go this route because of user confusion. Look into using a separate post form for front end blog posting. There are many topics here about adding a front end blog posting form.