I'm pretty sure they refer to the inability to type ß. Which is a real concern for type writers, less so for most keyboards. I'm writing this using an American keyboard and I don't see why I shouldn't use "ß" just because it's a different key(-combination). I also wouldn't start writing "spaet" just because "ä" takes a bit more work (Alt+U, A).
"Fehlt das ß auf der Tastatur eines Computers oder einer Schreibmaschine, schreibt man dafür ss."
Translation: "If the ß is missing on a computer keyboard or typewriter, you have to write 'ss' instead".
[0] http://www.duden.de/sprachwissen/rechtschreibregeln/doppel-s...