[cctalk] Keyboard Blockers?

I picked up a keyboard for a Wyse terminal at a flea market the other day.  When I tried some of the keys, they couldn't be depressed. I thought, at first, some dirt or debris had gotten stuck there, but on closer look I saw something black below the keys that seemed to be stuck.  I pulled a key cap off and found a U shaped piece of black plastic that was put there on purpose to prevent you from depressing the key. The question came to mind; "What sort of application would be so crude that you would have to prevent the user from depressing certain keys?" One of the keys was a Break key, which sort of made sense to me, because it would halt a PDP-11 if that was the host machine. This was the first time I had ever seen this kind of thing, was this common long ago?