As an online merchant, two things happen. A. We get paid, and B. We ship the product. Once we ship a product, we have virtually no way of getting it back. Chargebacks kill us because they are one-sided. Don't add them unless you can guarantee that we'll get back the product we already shipped.
Right, so you wouldn't ship a product until the chargeback window had closed. But the advantage would be if some guy calls you up and says "I sent you 50 bitcoins, where's my XYZZY?" you say "I have no record. Charge it back or send us proof
we accepted your payment."
Or, "I accidentally sent you 100 bitcoins instead of 1.00, here's the record, send them back to this address". What do you do? You don't want to steal from someone who sent you money by mistake, but you don't want to risk the next call asking you to send them back to some other address.
This type of chargeback is a great idea. I recently posted a suggestion to add support for them to bitcoin and to make transactions truly anonymous at the same time. (Admittedly, it also had some downsides.)