Quoting the Crisis

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