Reason:
Because I give up trying to fix these.
These are constantly confused with each other, a lot of people mix them up, and our current meaning of black and white is technically within the range of greyscale anyways. In other words, technically speaking black_and_white should imply greyscale.
In real world usage however, the term "black and white" is very commonly used as a name for certain "non-colored" things like old photographs and televisions, which would technically be greyscale by our usage. This is causing a good bit of the mistagging, but I've seen them both ways.
Lastly, I don't see enough reason to make a distinction here anyways. The purposes of the tags overlap since they are a way to refer to "non-colored" art, regardless of the technical differences.
Semi-related: I wanted to request a convenience shortcut alias bnw -> black_and_white/greyscale if that's alright (it's unused at the moment, but I thought I'd ask). Never mind, baw should work fine.
Updated