Couldn't Amazon just say the same thing? 'Not my problem, we offer full refund and if that isn't good enough you can take your business elsewhere'

No. You can’t “not my problem” your way out of being an accessory to fraud.

Why not? It seems like getting a full refund is a pretty big mitigation.

What other crimes do you think should allow the “not my problem” defense? A crime is still a crime even if full restitution is made.

There's a pretty long list. If someone sells a fraudulent product by mail, I don't hold the mail carrier accountable. If I buy a product I don't like on Craigslist, I don't hold them accountable.

The mail carrier does not advertise the fraudulent product and does not handle the financial transaction. There is a pretty obvious difference between a mail carrier and Amazon. Craigslist actually is responsible for stuff that is offered on their website, see also the long-running debates about sex workers advertising on Craigslist.

Craigslist is responsible for sex stuff only due to a very specific and stupid law.

It is not only the customer who suffers damage, but also the seller of legitimate merchandise who is not compensated by Amazon for fraudulent sales. Also, Amazon 100% has a spreadsheet somewhere calculating the share of customers who don't bother claiming fraud compensation.

Do you think Amazon cares? If most customers keep coming back, then Amazon is happy. If they have enough sellers too.

Do you think it's ok if I steal from you as long as I promise to return whatever you notice?