Just like most things in life, it all boils down to money and if I had to bet, I would say big money. Let's run a scenerio here that google has to (probably already have) consider. I have no clue if my estimations are close to accurate.
Let's say there are 50,000 people/organizations activly doing arbitrage in the world (again I have no clue if this is close). And lets say those people spend on average of $200 per week on the adwords network. 50,000 x $200 x 52 = $520,000,000.
If google stoped this practice, they would loose $520 million over night under this scenerio. There would be no reason for these people to have an adwords account and spend $200 per week with no return. (again, play with the numbers any way you want, but even if you think my number is too high by 75% then now we are only talking about $130,000,000 only!).
I am sure google hates this practice. But they have already proven they will sell out for a few dollars (see the Chinese). Just like it might be considered easy money to the arbitrage players, it is also very easy money to google and their high stock price.
I am sure Serg would hate to answer the stock holders about why the earnings are several hundred million dollars light next year.
