Section 6(g) of the Act lets the FTC make rules to enforce Section 5's ban on unfairness. Both are good law - 6(g) has been used many times (26 times in the five years from 1968-73 alone!).
The DC Circuit court upheld the FTC's right to "promulgate rules defining the meaning of the statutory standards of the illegality the Commission is empowered to prevent" in 1973, and in 1974, Congress changed the FTC Act, but left this rulemaking power intact.