Makes sense because Alibaba is outside the influence of the US Government.

Amazon would come under a great deal of pressure from the US government if Bitcoin or other alt currency were to be accepted.

Amazon probably is facing a day when they will come under scrutiny by the US as a monopoly anyway, particularly if their acquisition of Whole Foods is a success and they disrupt the Grocery industry.

Accepting crypto would probably accelerate this timetable and would cause Amazon to be compelled to break into smaller (regional?) companies.