How is that I never drew the connection between on/off and binary! So simple! I must be an engineer.... facepalm
I see that as a usability issue. I'm willing to bet that IEC did no usability studies. Compounding the issue... did they do any public-awareness campaigns? If so, they all missed me :/
Related issue: Menus (my Apple devices/software seem particularly acute) that don't tell you a current on/off state - but expect you to deduce it from the fact that (apparently) the option to shift to the other state is available (non-greyed-out).