Why was I asked by the seller to cancel my Amazon Marketplace order, instead of them canceling it themselves?

I ordered an item on Amazon Martketplace, and shortly after I received an email from the vendor (via Amazon) advising me that the manufacturer has reported a fault with the batch so they are cancelling the shipment.

However, in order to cancel the order and get a refund the message asked me to request the cancellation myself by messaging the vendor with a screenshot of the email.

The email was genuine – the links all point to genuine Amazon pages, not a fake or anything – and the order is now showing as cancelled in my Amazon account. The payment is still pending in my bank account so I presume it will be cancelled eventually.

So my question is why didn’t the vendor cancel the order themselves from their end? Is this simply not possible on Amazon Marketplace (seems unlikely) or is there some negative consequence (fees, lower rating) the vendor is trying to avoid by me cancelling the order myself?