So you might say then that Amazon.com customers would be angry if Amazon withheld, because transactions would be slower (they would wait until one of Amazon's miners solved a block). But I have several rebuttals to such a rebuttal.
Main rebuttal is Amazon could accept 0-confirmation transaction and let the withheld data sit on its own miners until they win a block solution. The customer wouldn't notice.
I have other possible scenarios for how the attack could be done.