They're avoiding the other elements of the law. It institutes a few other things besides just salary transparency. Companies also have to maintain a record of salaries/wages in order to prove they didn't discriminate based on sex and they have to announce advancement opportunities to existing employees (something many companies shy away from).
It should be, but companies, like people, aren't as rational as we'd like to believe them to be. For whatever reason there is a tendency to keep people who aren't on leadership tracks (which often means getting picked by a leader above you) where they are and with minimal pay raises. But then they hire new people at higher rates (because you have to, to get them in the door). This law makes that much harder to accomplish without creating conflict. You announce a new job paying 20% more than your current employees get in that position, they'll know they're getting screwed.