People may not agree to engage in gladiatorial contests in which they fight to the death, to sell themselves into slavery, to agree to have their knees broken if they default on a loan... It seems that the prohibited transactions are prohibited because they are highly offensive to nonparticipants. Why they are highly offensive remains to be explained.

These prohibitions seem to me to be akin to collective bargaining. If some employees agree to work under very onerous conditions, they weaken the bargaining position of other employees who hoped to negotiate better conditions. Therefore the latter exert social pressure, creating unions and labor laws to discourage anyone from accepting a too-onerous bargain.

Similarly, the mass of people who may one day borrow money are opposed to anyone agreeing to have their legs broken by loan sharks. By passing laws to limit the enforcement capability of lenders, we prevent high-risk borrowers from getting any loans but reduce the interest rates on our own lower-risk loans.