I think this is merely due to the fact that the definition of "OS" varies from OS to OS. In the GNU/Linux world, the "OS" generally refers only to the kernel and the minimal set of command-line utilities necessary to get a basic system up and running, whereas in Windows and OS X the definition of "OS" also includes the desktop environment and many programs that come installed by default. If Windows or OS X were to be split into a desktop environment and kernel, then their support for these image formats would almost certainly end up in the desktop environment part.

If a KDE/GNU/Linux distribution installation comes with first-class support for these image formats, I don't see that there's any reason to disqualify it for having the image support in the KDE part of the installation rather than the GNU/Linux part.