They don't want the tax headaches something like that would cause. It would be a continuing nightmare, considering how many countries they'd have to deal with. Basic payments are enough for them to deal with.
Like all American businesses, and all American individuals with any sense, Amazon lives in terror of the Internal Revenue Service. It only does business that it can prove did not involve money laundering, criminal activity, terrorism, etc etc etc. That excludes paying royalties with gift certificates, PayPal, Bitcoin, and other ambiguous forms of currency. (An exception is the gift certificate option on Amazon Associates.)