When minimum wages were proposed, it became clear that the idea of all races being paid equally was not going to fly. But since servants were mostly of color, those who provided service could be exempted from minimum ages, and became reliant on the generosity of the superior for most of their wages.
That way service providers were still at the mercy of the served, the served were superior in the relationship, and groveling was required to make a living.
Tips are a relic of American racism, which is why most other countries don't have it.