Ok, let's scrap the high-tech detector with enough resolution to tell you're
moving your hand and take a more realistic one which can just tell that
you're sitting at the computer -vs- being somewhere else in the room -vs-
the room being empty. The voice can tell me a lot while my feedback is
very limited.
Or, take your common dumb temperature control: you have a box with two
buttons and a simple LCD display which can show only two digits. Yet,
nowadays everything tends to have a chip inside -- and if anything inside
has anything to do with GPLv3, you suddenly need to convey the legal
notices...
My point is that the requirement of every interactive interface having a
feature to display the legal notices can be a severe use constraint.