Abstract away from the people. At the time, it was simply not in Facebooks best interest to enter into such an agreement - at their ridiculous growth, they got lots of money and needed to put it into top talent, which means they need to aggressively recruit. If you then enter into an agreement not to recruit from many top companies in the valley - that hurts your chances.

It's not that Facebook is the only ethical company in this, it is just that at the time there was no upside for them in it.