Possibly? Or they think that their payments are required to be ranked fairly on Google's service and view it more as blackmail than anything else. Searching for "amazon" brings up an Ad paid for by Amazon, searching for "books for sale" brings up paid Ads for four different book sellers - if any of them stopped paying google they'd be, at best, the fourth result - and the first Ad result is for the first organic result, the next organic result is the fourth ad, the one after it a sub-page in the last result and the fourth organic result is craigslist which didn't pay for an Ad. In fact, Amazon isn't even on the first page of results except for their Ad.