...I'm not sure I could quite imagine why anyone would choose the SA license considering even if they included the sprites under SA for their own CC BY SA project, they wouldn't be able to stop people from using my original cc by license.
The point is that we can't practically maintain multiple different licenses for art in this project. You don't need to publicly advertise that you offer(ed) the images under CC BY SA, but we need you to release them to FreeOrion under CC BY SA so that they can be re-released with FreeOrion under the same license as the rest of the art.
So in effect there'd be no way to enforce the share alike.
Since you're already releasing under CC BY (non-SA) there'd be no need to enforce either, but actually enforcing the SA part of the license for your art isn't the point of the request.