The reason being is that they can get a better deal on group health care coverage that way.
Man, is that a slippery slope - because once you allow your health insurance company to push you into baring something for one perfectly legal health risk activity you open the gates to all personal activities that are a health risk.
For example, who is more likely to get an STD - married people or single people? You could certainly cut your health care bill by refusing to hire anyone single.
For that matter men who have sex with men, according to the CDC, are much more likely to get syphilis, so I guess you could stop hiring all gay and bisexual men as well.
Heterosexual women, under about age 40, have a fairly high rate of getting pregnant, another expense for insurance companies, so maybe they could save money by not hiring them.
Meanwhile diabetes, an expensive chronic condition is 60 percent more likely to occur in African Americans than whites, so there's a ton of money to be made by not hiring them either.
People who drink beer at night are more likely to have liver problems.
People who eat a lot of pork products are more likely to get heart disease.
The list goes on and on, but the only eventual outcome I see is a workforce that contains nothing but white married men and partnered lesbians who are Jewish or Muslim teetotalers.