It's not really illegal to pay women less than men for the same job. It's illegal if the reason you're doing so is because you're discriminating against women, but outside of catching an email trail or getting a company officer on tape saying it's policy to pay women less, it's very difficult to prove. If I hire two candidates for the same position, one man and one woman, both of whom are equally qualified, it's not illegal for me to offer them whatever I want to offer, and if I can get one for half as much as the other, it's legal for me to do so.
Companies whose policy it is to offer women less won't ever be caught actually saying that's what they're doing. And given the amount of power employers have in this country, it's practically impossible to bring a successful suit, let alone criminal prosecution, for discrimination. If an employer claims that he judged the worth of a female candidate to be lower than that of a male candidate, provided that judgment had nothing to do with the sex of the candidates as such, a huge amount of evidence has to be produced to prevail against that employer--and that's the case even when other reasonable people might see no difference in the qualifications between the male and female candidates. Employers get to count intangibles such as "attitude," "aggressiveness," "professional tenor," and so on as qualifications, and can claim to have based any difference in pay on the employer's comparative perception of those qualities.
I know all this, because I've worked for companies that had unwritten rules about who got raises and who didn't...only in both cases, it was women being paid more than men. Nevertheless, given the statistics, I think we can guess that more companies discriminate against women than against men.