We don't.... but education has gone severely downhill, and people no longer understand the meaning of words nor the use of a dictionary.
Referring to motorless bicycles as "acoustic" is just like wearing a sandwich board that says "My education is woefully embarrassing." In some ways, this is convenient, because as soon as you hear someone call a motorless bicycle 'acoustic', you know that you can safely ignore everything else they might say.
It was a joke. Everyone got it (almost). Geeze.
What's a "motorless bike?" Is that an actual term, or something you made up? Googling doesn't bring up any hits on the term. They've always just been "bikes" to me.