The legal question is definitely interesting, but I don't see how it would be unethical to use the most powerful set of signals available. The situation seems to me a bit similar to how grocery stores and cafes package what's essentially the same product under a large range of price points to let the more price-sensitive customers self-select.
If I find anything unethical, it's knowingly continuing the practise of writing job postings that put one group of people at a clear and documented disadvantage.