Got an interesting email today:
Hello from Fulfillment by Amazon,
Starting September 1, 2019, Amazon will automatically make eligible inventory available to US charities when you choose to dispose of overstock and returns stored in US fulfillment centers.
This program will reduce the number of products sent to landfills and instead help those in need.
The donation option will apply to inventory that you select for disposal manually and through automatic disposal settings. Our usual disposal fees apply. You can opt out of the donation program at any time by disabling FBA Donations in your FBA settings.
For more information, including the organization handling the donations, visit FBA Donations program.
The Fulfillment by Amazon team
I always suspected that some inventory was simply discarded rather than being sold, for a variety of reasons, but it seems I may have underestimated the volume of such disposals.