They can clearly can but the only problem is do merchants on amazon will accept this update and changes issued by the amazon company if they issued bitcoins as one of payment methods.I think that some merchants will not accept it
Who cares whether the merchants accepts bitcoin or not ,if they let users to purchase the products with bitcoin they could cash out immediately and there is nothing worrying about the volatility we have with bitcoin if they could cash out immediately and they could come up with a good front end.