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Suppose Google wanted to catch people that were displaying ads that weren't AdSense/AdWords ads, but claiming that they were? Perhaps the impostors wanted to add an air of legitimacy to their non-Google ads? Or maybe they were rejected by the AdSense program and are trying to make another ad program look more prestigious?
By changing the "Ads by Google" line to "Ads by Gooooogle". They could search their own indices to rapidly find people that were using the "Ads by Google" phrase and make them stop. They could have done this before, and then cross-referenced the pages with their "approved" AdSense sites/pages, but that would have been a monumental job. This makes it a lot easier.
Just a thought.