I don't think we can blame any of that on the difference in usage between groups. The problem is that BB was arbitrarily chosen as a cutoff for expertise (through marketing and movies, mostly) by much of the public and even many within the MA community. If you had a BB, you were an "expert". And not until then.And that ambiguity on the meaning of the black belt is part of what enables bullshido in the eastern arts. I think the lack of belts or other insignia helps keep "dim mak" and "no touch knockouts" out of boxing and wrestling. Ultimately, I think the spectator sport aspect of these arts play the largest role, but the lack of belts doesn't hurt either.
It's a bit like assuming only someone with white hair or a bald head can have wisdom. It's arbitrary, and the fact that different people turn grey or lose their hair at different times isn't really the issue.