Red Sox cannot poach Cardinals' pitchers during the World Series because there are collective bargaining clauses between the league and players associations to bar that from happening. But Red Sox can sign any free agent from Cardinals when they are available.
There are no such collective bargaining agreements between employees and employers in Silicon Valley. You are working with "at will" agreements which can be broken at any time. There is nothing stopping anyone from poaching other company's employees, sans collusion.