Arguably so is threading (except internally as required for seamless cross-platforming) and networking definitely is, yet SFML provides those.
Everything which is in sfml-system is not purely related to multimedia, it's more a core package on top of which every other package is built.
Network perfectly fits in a multimedia library. If a medium is a "storage and transmission tool used to store and deliver information or data" (wikipedia), then network is definitely one
x.x
Copying is used in most graphical applications, from text editors to image manipulation programs to video games to web browsers. So I would certainly say it's not at all a bad idea to provide clipboard functionality.
So as tons of features related to GUI and windowing. But I can't make SFML as rich as Qt or wxWidgets. Even if I wanted to, I couldn't, there are so many things that have to be done before thinking about such things.
Well I wouldn't want that, either. I like the freedom to develop my own GUI and have it work internally exactly how I want, and without bloat added so that everyone can have the GUI work exactly how they want.
However, clipboard is not really related to GUI at all... I'm not expecting SFML to handle copying and pasting for me, and definitely not how to use information on the clipboard (IE, rendering an image).
Just how to add data to the clipboard and get data from the clipboard, which follows similar semantics on every system and would be a very small addition to the library.
std::string text = sf::clipboard::getData();
...
sf::clipboard::addData(text);
It really just needs to be that simple.