Because they may be the only work in your field or the best paying work in your field that is present.  If a Nazi Skin Head has to hire fairly because of the law for his bank, and he is the only Banker within 20 miles out in the mid-west, the black employees should not have to give up any benefits or be kept from company funded gatherings just because the Nazi owner does not want to pay for blacks to come as a matter of upholding his religious/political beliefs.  People can only find work where work is available & not all work is necessarily available outside of these businesses.  Say like with the only hospital in a town perhaps which happens to be Catholic run. 

Basically, if you are funded in part, by the government, such as a college or hospitals & therefore you must hire non-catholics who are qualified for the jobs you offer in order to keep that tax funded support then you must afford them the same benefits working outside your business offers or choose to give up the funds.