Google makes most of it's money from Adwords on the search pages, Adsense on content pages only brings in a fraction of the income.
They only make money from Adwords if searchers continue to trust and use their search engine. If people start to think they don't deliver the best search and go elsewhere, the main income gets threatened.
Sounds like they've taken a strategic decision to sacrifice some Adsense cows, to protect the Adwords golden cow. Which is smart, you have to hand it to them.