Because they are making money from him - Buying traffic from one place and sending it to another is just business. Spam is a different matter altogether. As long as he delivers value to the advertisers then there should be no problem.
i don't know if this has been posted before... if it has been, skip my message i just found a nice ShoeMoney video: http://www.alsuran.net/2006/12/10/shoemoney-video/
If shoemoney can advertise on the open that he is cheating Google, I wonder why Google has not banned him.
once again look up and see what arbitrage is. It's buying something in a market that is cheap and selling that same something in a market that is high. Let's not forget that google gets paid 2 times on this transaction. and the publisher pays once and gets paid the second time.