The US Constitution does not prevent employers from discriminating for any reason whatsoever. The constitution was only written to ensure government did not discriminate against citizen, i.e. equal treatment under the law.
Once we started meddling in this stuff during the civil rights era, we opened the door to constant conflict over which rights are more important. Does religious freedom, for example, force employers to hire employees who cannot properly perform the job requirements?
This stuff was so much easier when the government stayed within its constitutional bounds.
This is why more and more employers have figured out that the best business practice is to avoid hiring anybody.