Adding a comment box on a page, not a blog.

Yes comments are only for blog posts. SS could of course add comments anywhere but I assume that blog pages are the most common use case for comments and the numbers of users who would want that feature outside of a blog context just aren't there.

You can of course use an outside provider for a comment feature.

Disqus has been a long time provider of such a service. Now keep in mind that this may not be plug-and-play. You may have to work out how to integrate it into an SS site for your particular use case.