The answer is federal and state legislatures. Milady draws the distinction directly: laws are written by both federal and state legislatures, which determine the scope of practice and establish guidelines for regulatory agencies.
The book pairs this with an equally important second half of the same idea: rules, by contrast, are written by regulatory agencies (like a state barber board) to explain how to comply with the laws legislatures pass. Regulatory departments, health agencies, and state boards all write rules under this framework — they don't write the underlying laws themselves.
Milady Standard Barbering, 6th ed., Ch. 4 (Infection Control), p. 62: "Laws are written by both federal and state legislatures that determine the scope of practice and establish guidelines for regulatory agencies."
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