Probably because the vast majority of EOS camera owners don't know their mount designation is EF/EF-S. All they know is that their camera says EOS on it, and that's the lens mount they ask for. I've run into that many, many times. It's not just a Canon issue either.
Canon's mount is called EOS, not EF(-S). EF merely stands for "electronic focus", and there are some EOS-mount lenses that are not designated EF, like their tilt/shift lenses and the MP-E 5x macro lens.
Splitting terms.... are we? or just adding to the confusion.....
EF is designation for autofocus lenses on Canon EOS system. EOS being a term marketing folks at canon used. No other manufacturer of lenses uses the term EF to refer to lens mount design. As such, when people refer to the EF mount they are referring to the mount used within the EOS implementation. The MP-E and TS-E lenses are still technically EF lens mounts BUT simply do not use signals nor motor to focus (manual only).