Appear multiple times on a page load and you get a nag when running the tools. Most all are very small gif files but it still gets a tick mark on the tool
This is not appropriate usage for easy customization of Post Icons, Graemlins, Avatars, Forum Images, News Images, and Moods, where a forum owner would want to replace one or two images with their own custom image/images to fit their forum presentation. Especially if a single page was displaying 5 of sprites from a package of 100. All of those colorful 100 sprites would need to be downloaded when just 5 of them were needed.
Icon fonts are similar to sprites in that the achieve the same thing: combining multiple images into a single request. https://fontawesome.com/icons?d=gallery
These validation tools are created for the vast majority of cases; this for example would be a recommendation for static content websites where things seldomly change and the users don't have access to multiple layouts, and not web forum where there are different styles, moods, icons, avatars, animations, etc are involved.