I guess, when Bitcoins are more mainstream and the connection to the dark net is not so strong anymore, they might accept it. I don't expect this until 2020 but maybe later. More and more small shops are already accepting it. This is how it starts. Give it some more years. 
So amazon is not still accepting bitcoins because of the situation that it has been connected and being used in the dark net. I think that is not fair at all.
Because even the US dollar is being associated and used for different type of illegal activities. But I think if more and more people are going to suggest acceptance of bitcoin.
Amazon is going to accept it nearly in the future.
I dont think thats the issue why amazon still didn't accepting bitcoins for now, since it cannot do nothing with darknet to their payment option and i think the reason why they still not accepting it for now is due to its flactuation and no guarantee users for it globally,
But if amazon will see some stats and analysis for its potential provabky bitcoin will immediately be added without further more hesitation on their site.