If 20 people are selling the exact same item, the info that G shows the shopper is not about the Sellers, but it is rather about the Item itself (regardless of which Sellers listed it).

In this regard, the (composite) Listing belongs to Google, they use images (and more) from everyone selling the same GTIN / MPN / Brand combo.

So if twenty sellers list the same item, Google uses all of their resources to create the composite listing.
This is one reason why you can't use watermarked images.

Since you are adding these images to your shopping feed (even if you're doing it just to get organic traffic in your stated case), the resources are tagged to the listings (both paid and organic).

I think this is among the many reasons why G has the free listings, it encourages more feeds, and richer feeds.

And I believe it's probs in the Feed TOS, so I don't think anyone is doing something you didn't agree to (I am guessing here on this point).
See the part here ( [searchengineland.com...] ) that says...
"To get started, you’ll need to open a Google Merchant Center account and upload a product feed. Sellers must opt-in to “surfaces across Google” to be eligible for organic visibility,"

In the scenario you described, I do not know of a way to stop it.

Thanks for the reply.

We have some products that are out of stock, so these automatically get removed from the feed - but it still happens with those products too. Google are using data from other sources beyond just the feeds.

Some of our second, third, fourth images do actually have our logo on, and these images too appear in the onebox to help promote other sellers.

g's algos are not smart enough to know they are ripping off copyright stuff...

Then again, g seems to think anything they list is theirs. :(

ymmv

...why G has the free listings...

when the service is free, you are the product.

(or in this case your provision of "free" rich product descriptions, with relevant images, is "the product")