English Speakers Need Not Apply

I have seen Help Wanted signs posted on the windows of several restaurants in New York advertising openings for a delivery man or a dishwasher but in Spanish only. Such signs give an English-speaking job seeker the idea that they need not bother to apply. To me this is discrimination. If it is against the law to discriminate on the basis of race, sex, color, national origin, etc., then why are these signs permitted?