They want to streamline the job posting portion of HR. They don't want to have to worry about whether or not they have to post the salary range, so they just do what the most demanding law they deal with requires.

Now they only really have to deal with areas that have laws that contradict with laws in other areas. Then you'd default to the law that benefits you the most and deal with the contradictory areas explicitly. Since you have to do the work anyway.

For example, let's pretend that California had a really stupid law that forbid salary ranges from being posted on job listings. Now Microsoft has to be careful about how and where they post jobs. And since it's beneficial for them to hide the information, they'd likely only post the salary ranges where they were required to.

But absent that, don't do work you don't have to do.