I was in the defendant class for one of these Silicon Valley film anti-poaching lawsuits. :) And I agree that companies, including the one I worked for, shouldn’t be able to agree to prevent hiring from each other. The main problem this caused was it made it harder for any individual to leave one company for another, with no poaching involved, and because of that (it was argued) it suppressed the general competitiveness and salaries of the employees.
Do keep in mind that this case (companies agreeing they won’t hire from each other) is a bit different from an angry employee leaving and soliciting some friends to go with him to a new company. Like I said, it might be perfectly legal, and yet still cause you serious trouble if you do it. Some people have ended up a lot more frustrated after leaving because of things they did on the way out. It can work out well if you’re really in a position to start your own company and fight like the Traitorous Eight, but for most devs who just want a job, making huge waves on exit is pretty dangerous. Don’t take my word for it though.