… is from page 213 of the 5th edition (2015) of Thomas Sowell’s Basic Economics:
An employer who refuses to hire qualified individuals from the “wrong” groups risks leaving his job unfilled longer in a free market. This means that he must either leave some work undone and some orders from customers unfilled – or else pay overtime to existing employees to get the job done. Either way, this costs the employer more money. However, in a market where wages are set artificially above the level that would exist through supply and demand, the resulting surplus of job applicants can mean that discrimination costs the employer nothing, since there would be no delay in filling the job under those conditions.