Amazon is very big company, it is the largest Internet-based retailer in the world. They make a lot of money, and they do business in many countries. Probably they pay big taxes everywhere, and with bitcoins it can be a little tricky. First not all countries have regulation about bitcoin usage, how they will show their profit with bitcoins in country that not have law about it?
I think its not so simply to implement bitcoin such big structure, and that is the only reason why they still didn't do it. But now when New York accepted bitcoin, maybe amazon will offer to people from NY to purchase things with bitcoins, we will see.
Before any auction being taked byt the countries around bitcoin, google were allowing people to purchase some products with bitcoin, with a third company providing the money to them and taking the bitcoin, soo there isnt nothing to do with rules from others countries as a similiar act may be adopted by amazon, and now sure they may allow it a NY and them to all, but its just they wanna to work with, that are some companies providing those service between the costumer and the company.