Actually this recommendation makes a lot of sense. You generally have 30 days to return your purchase, no questions asked. The algorithm knows that you're now quite likely to buy another lawnmower/audio receiver. Perhaps there is a fat chance you may even spend more the second time over (after returning your previous purchase)?

Since Amazon knows whether you’re submitting a return.. they could just filter to that scenario and then provide such recommendations.

There's a good chance you purchase the 2nd one before returning the first.

Or you sell the 1st elsewhere like Craigslist.

Either way, the numbers show those ads (remarketing ads in industry speak) are insanely effective.

They might be weighting by E(rev|ad_shown) in which case you'd only need a very small amount of repeat users to make it worthwhile to show it to everyone.

Not that that justifies the practice.