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.
Deuteronomy, chapter 22
13If any man take a wife, and go in unto her, and hate her,
14And give occasions of speech against her, and bring up an evil name upon her, and say, I took this woman, and when I came to her, I found her not a maid:
15Then shall the father of the damsel, and her mother, take and bring forth the tokens of the damsel’s virginity unto the elders of the city in the gate. . .
20But if this thing be true, and the tokens of virginity be not found for the damsel:
21 Then they shall bring out the damsel to the door of her father’s house, and the men of her city shall stone her with stones that she die: because she hath wrought folly in Israel, to play the whore in her father’s house: so shalt thou put evil away from among you.