"In the Zone System (for B&W) - why is the colour of the spot metered area unimportant? ... Why isn't this concept mentioned anywhere?"

The majority of zone system books address your question during the discussion of exposure meters. The answer is not always straight away, but generally state light meters do not know the subject matter nor its colour. The light meter wants to make the subject, regardless of colour, a middle grey.
Therefore, from the lightmeter's point of view, the colour of the subject is unimportant. However, our individual interpretation of the subject's reflective surfaces is what is important.