Yup. Same law that says you can unionize says they can't stop you from sharing pay and benefits information.
The law says they cannot; However, most employers feel the law is unfair to the employer and may very well intentionally disobey the law in a subtle manner.
If they find you shared your salary, then your company might find another reason to fire you and terminate you for that other reason. In an at-will state it's easier..... "According to the latest performance review, you're just not a good fit for our company, so we have to let you go."
Google could technically do the same for everyone on that spreadsheet. Sharing their own salary info would not be mentioned on the official papers as reason for termination, But their accessing/showing the spreadsheet could be grounds for termination upon suspicion of gaining unauthorized access to HR systems.
Companies need to make the money, and employment costs going up would be a huge negative for the shareholders and managers' bonuses.
I've seen this happen in companies that I have worked for in the past. There was one employee who was actively trying to convince the technical operations department (in a CATV company) to unionize. A Senior VP (known for his anti-union stance) came for a visit and spoke to the department strongly encouraging them to not follow a vocal minority. Within a few months everyone known to be involved had been fired for violations of company policies. I had a supervisor who actually told us in a team meeting that if he heard that we were sharing our pay information he would do everything in his power to fire us for cause. When someone on our team went to his manager he denied the statement and explained it away. His nose was brown enough the manager bought it and ignored it. Not long after that he started picking off team members one by one for various reasons, starting with the guy who went to his manager. It took him nearly a year but got around to me as I wouldn't play by his rules. I didn't care what anyone else made, I just knew my supervisor was ineffective and made that clear, although not directly. I was told that after me he went after another senior tech, the supervisor was shown the door rather quickly, that tech had friends in high places.