Amazon back charges the vendor for every return --- which likely includes return shipping and handling. I can assure you, Amazon doesn't just "eat" these costs.
Remember, Amazon holds the cash for all sales. The cost of returns is extracted from vendor disbursements. A fake card sold for $10 may cost the vendor $20 if it gets returned.
If enough people return the item which again, with microsd cards, is likely not nearly often enough on Amazon.
If “the system worked” i.e. companies get appropriately punished for selling bad products, then this issue wouldn’t still be so widespread. It’s basically a feature of Amazon now. People just assume they’re going to randomly get junk. It’s baked into our expectations at this point.
Remember, Amazon holds the cash for all sales. The cost of returns is extracted from vendor disbursements. A fake card sold for $10 may cost the vendor $20 if it gets returned.