"This scaling has absolutely no effect on the images within the post."
If you mean images already inserted, of course it has no effect: what a post displays depends on its sourcecode (which normally includes the dimensions of the displayed version), and its sourcecode doesn't change unless you edit the post. Such a scaled down image will change in the post only if you have previously deleted the width and height declarations in the HTML editor.
But if you insert an image after you've scaled it down, and select the Full Size option, you'll get the scaled-down version in the post.
When you scale down an image in the Media Library, you'll notice that you get a (permanent) Restore option - which is why the image remains stored at its original size. That's wise, in my opinion, for a variety of reasons: for instance, inexperienced users might find that they've downsized the image too much; or you may later need to change to a larger version if you switch to a wider theme.
But anyway scaling down an image in the Media Library is a poor substitute for what you can and should do in an image editing application: images resized by WP lose quality.