What's beyond me is why they can't find a minimal effort solution that still nets them a minimal profit or at least costs them less than disposal. I'm sure they're paying to dispose of this stuff.

Around the US, Goodwill has what it calls "outlet stores", where anything that didn't sell at Goodwill (a low bar to clear) gets sent and dumped into giant bins to be sold as-is by weight, as a last chance at redemption before the landfill. Despite the low quality of the goods, the stores are usually packed, and even Goodwill rejects have value. Before the pandemic my wife and I enjoyed the cheap entertainment of picking stuff up for a song and reselling it on eBay for fun.

I for one would gladly sift through bins of Amazon returns and buy them by weight. If you have "valuable" waste like this there's no need to ship it, people will come to you.