Usury laws are bright line rules that allow for business planning. We can try to police the same problems on an ad hoc equitable basis using doctrines like unconscionability, but these fuzzy standards make it very hard for businesses to know what is allowed and what isn’t. It might chill socially beneficial behavior, and it might also result in inconsistent results. While a business might prefer to operate with zero regulatory constraints, as between a bright line rule and a fuzzy standard with substantially the same substantive result, I would think that the bright line rule would be preferred.
— Adam Levitin in Usury Laws Are Dead. Long Live the New Usury Law. The CFPB’s Ability to Repay Mortgage Rule„