Because the two summaries should be *different* in most cases.  The first line of the checkin should ideally be a single sentence of less than 80 characters, while the NEWS item will almost always be longer.  The NEWS item should be a complete summary, while the first checkin line is a 'summary in context' (ie: it can assume the reader knows the files that were changed, since log -v will show you that).  This obviously requires that the succeeding paragraphs expand on that minimal summary (in most cases) in a way that doesn't look like a NEWS entry.

Now, this is obviously my preferred style and others may have different styles and/or disagree, but I'd like that style to be supported.

Note also that a big part of putting the NEWS entries in the tracker is so that they can be *edited*.  Populating them initially from the checkin message is OK, but I predict that the actual workflow will be to create the NEWS entry in the tracker as part of getting the patch to the 'commit ready' point, which means that having a tool that pre-populates the commit message from the tracker NEWS entry will probably be more useful to most people.  (And yeah, I'm sure that means a lot of commits will just have the NEWS entry as the commit message, which will make me sad, but oh well :)