"Ms. Mayer called a meeting with senior executives in August and asked them to sign a written agreement to stay with the company for at least three more years, according to two people familiar with the meeting." Is such an agreement enforceable? Or is this some meaningless pledge? If you want people to stay, give them a decent financial incentive.
It's not enforceable. In the US, you can't enforce an employment contract if it involves forcing someone to work for you. This was partially legislated as a legal defense against indentured servitude.
Well kind of. Technically you can't force them to work, but you can force them to pay someone else to work in their place. Most teachers have this kind of contract. They are not at will employees. They can't be fired during the school year and they can't leave either. If they don't show up to work, they have to pay the school back for the part of their contract they did not fulfill to pay for the substitute teacher. (This has nothing to do with tenure, BTW, it's true for tenured and untenured teachers alike).