Great question because this is a very rare use of an adjective as a noun, but it is different from most other nominal adjectives (as in "The poor are a problem.") Rather, it is a very rare use of an adjective to take the place of the entire concept for which it stands rather than to describe any particular people or things. Another example of this would be a book about jazz entitled, "The masters of cool." (normally we wouldn't think an adjective could be the object of a preposition, but it can in this rare use of an adjective as a noun). So it is a small class of nominal or nominalized adjectives.