On one hand there are many anti-discrimination laws in US
to prevent employers from discriminating employees on the basis of sex, religion, caste, nationality and so. On the other hand, US Govt. is shamefully trying to preserve affirmative action. Employers (and many others) have to make special arrangements, such as providing special parking space near the entrance, for disabled people because of the Americans with the Disability Act.
Employers can "discriminate" when hiring if performance of the job so requires. On this basis, the famous Hooters restaurant can hire only girls as waitresses.
Heck, you even have to prove that industriousness matters, otherwise you can be prosecuted for discriminating against the lazy.