Well, it's true in California. It's been in the employee handbook of every job I've worked at. The handbook allows termination for anything said that may cause a drop in productivity. In many cases as a manager I found employees with "friend" status no matter what there station was often recieved more pay than others. If you knew that the laid back guy at your job who doesn't work as hard as you do made more money for less work how would you feel? It could possibly effect your productivity.
No I don't believe this is true. There are no laws surrounding this: I think it would be highly controversial to introduce such a law. Discussing pay does however cause problems at work and it isn't unreasonable for an employer to ask you to not discuss your salary with anyone other than your manager(s). If you go against the wishes of your employer you may face disciplinary action, or if it is particularly troublesome (seen as stirring the proverbial) you may even get fired. If this is further drafted into a legally binding contract it becomes a legal matter and can induce immediate termination of employment.
You're right the employee handbook is not law. The employee handbook at the places where I've worked or managed are distributed as rules and regulations of the job. If you read through and except the terms of the handbook and sign it within the 30 to 90 day probation period you agree and keep your job. If you do not sign it your position is forfeit. Sadly, alot of these types of books/contacts also allow for termination without question. The individual's signature agreement does make it a legal document though. The termination without question part of the handbooks popped up when the recession began. I've worked mostly blue collar jobs like factories, hotels, and warehouses so I don't doubt that the lower class workers and illegal imigrants are being mistreated. I left a management position a while back because of the mistreatment of my employees.
There's a lot of stuff like that in my current handbook/contract. Basically it's employers working around the law by putting questionable practices into a binding contract. Like in mine, I am contracted to 35.5 hours, but "the worker may be required to work more, or less, depending on the business needs". Basically this means my employer could forcefully give me very few hours, lowering my wage and making me leave my job. And with that snippet, it means I'm not entitled to overtime pay either.
Yeah, the crap some employers put into contracts is disgraceful. I get that some companies have good reason for making certain demands (such as needing you to work over 48hrs/week), but most of the time they're just trying to screw the employees. They really should be forced to justify such demands IMO.
Having said that, my employer in France was making what I considered unreasonable demands, so I just ignored them and didn't show up to work - you can always do that if you're in the position where the employer needs you more than you need them (it would have been pretty much impossible for a small shop to hire an experienced ski tech mid way through the season - esp. when the boss is an arsehole).