The answer is professional responsibility. Milady's history chapter states directly: by 1929, the AMBBA adopted a barber code of ethics to promote professional responsibility in the trade.
This sits in a broader timeline the book traces — the Associated Master Barbers and Beauticians of America (AMBBA) had already worked with the National Association of Barber Schools to standardize training, and state licensing laws followed shortly after in the years following 1929. The code of ethics itself was specifically framed around trade-wide professional responsibility.
Milady Standard Barbering, 6th ed., Ch. 1 (The History of Barbering), p. 15: "By 1929, the AMBBA adopted a barber code of ethics to promote professional responsibility in the trade and later published its own barbering textbook."
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