A few hours later, he stops by again and sees more men there, so he hires them as well. And, again, a few hours later, he does the same. So he tells them to go out and help the others finish picking. Now, Jesus isn't trying to teach us funny economics. What He wants, in fact, is for us to feel the same sense of outrage that these workers would have felt, so that we can recognize something in ourselves.
People are jealous by nature.
We are covetous, though we don't like to think so. We are so afraid that someone will get more than us. God is gracious. He wishes to give us good gifts, and thereby establish His rule in our hearts. But we are spoiled children.