Equal Employment Opportunity Act of 1972.  The first company sued under this law was United Air Lines.  The worst offender in the country was made an example of how not to treat people.  I worked for this company at the time.  It took only about a year for United to lose the case.  The second-worst offender to be prosecuted was SFO Crocker Bank.

Links:  https://www.eeoc.gov/eeoc/history/50th/thelaw.cfm

http://openjurist.org/475/f2d/216/spurlock-v-united-airlines-inc