It is a case where women are following the letter of the law but not the spirit. There is a big debate among Orthodox Jews whether these should be allowed or not, in fact, many women do not wear them precisely because of this reason. This view is more prevalent in Israel where women don't feel out of place wearing a head covering even at work. In the US more women wear wigs because many of them work and either can't come to work with some other kind of head covering or would feel very awkward, therefore they follow the letter of the law and wear a wig.