It's marketing. People here seem to be so tied to their this or that doesn't work way of thinking.
Bitcoin as an additional payment option doesn't cost anything if it isn't being used, but will very likely result in a wave of customers they otherwise wouldn't have.
Implementing Bitcoin support absolutely has costs.
They need to dedicate developer and server resources on an ongoing basis. They need to develop and maintain a robust wallet and custody solution, as well as a robust solution for currency conversion. They also need to account for custody risks. Or if they hire a processor like Bitpay, that's a 1% cost right there, which is only a tiny bit better than Visa. I also assume processors like Visa give giants like Amazon favorable fee arrangements as well, so Bitpay may in fact be worse.
I also strongly disagree that it'll "very likely result in a wave of customers they otherwise wouldn't have." What businesses implemented Bitcoin support and then saw massive growth? I remember several cases where businesses added Bitcoin then later dropped it because there was no interest. Adding Bitcoin as a payment option has never driven consumers to Bitcoin.
Besides, Amazon doesn't need to achieve growth that way. They are basically taking over the world by expanding their core logistics model and consolidating neighboring industries. As long as they offer all the major payment options consumers want, this is an afterthought to them at best.
BitPay alone did +$1 billion in conversions last year, where it's safe to say that the far majority of this is related to Bitcoin.
Give people the merchants they frequently shop at, and this $1 billion will become +$2 billion, and more as time goes by.
I'm pretty sure that Bitcoin holders intent on buying stuff on Amazon already buy gift cards with bitcoins -- Bitrefill, Bitpay, other gift card malls. If they use it directly on Amazon instead (or on Amazon through Bitpay), I just don't see it making a big difference.