they could sell them for uber cheap but would rather destroy them because otherwise it's a lost purchase at full price
Or they don't have the right to sell them, or they're stock that they've been instructed is unsafe/fake by the authorities, or that they can't resell because of hygiene issues, or they've been instructed to destroy by the lawful owners.
Given that Amazon do warehouse deals I imagine they're quite happy to sell it cheaper if they're able to, but there are a host of reasons why a retailer/shop front like Amazon may not be able to - for example if they get informed by Trading Standards that something they've got in their warehouses is fake or electrically unsafe and cannot be sold they've only got one option - dispose of it in a way that means it cannot end up in consumer hands. HRMC and Trading Standards routinely oversee the destruction of quite large amounts of materials themselves.
There are also times when it is, unfortunately cheaper to simply destroy stuff than even give it away, especially if it's low value/bulk items*, and other times when you legally cannot give it away because it is unsafe,
130k items a week from a distribution centre sounds a lot but it might be a fraction of 1% of what moves through that facility in a day.
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I'm not sure if it's still the case, or if it used to be in the US as opposed to the UK, but iirc there also used to be something where book inventory held by, or on behalf of publishers had a tax value and it had to be paid every 12 months, leading to publishers having a very strong incentive to pulp any books older than a certain age because not only were they usually dropping in value whilst requiring storage to be paid, but every year they had to pay tax on them with the result it wasn't unusual for them to have a negative value..
*I've been in a large DIY store where they had bags of concrete and plaster mix by the exit with a sign saying "free please help yourself" as it was past it's sell by date (plaster and concrete mixes do go off), in that instance it was still probably usable but not worth reducing and giving it away was the last option before they had to pay for disposal.
I suspect no reputable builder would have taken it up because a potentially lumpy mix that could ruin a job isn't worth saving a fiver, but for a diyer who just needed some concrete for a fence post, or enough to patch a crack it was probably ok.