It is a huge problem that will get bigger. It is one reason I'm not self-publishing three novels I have ready. I'm trying to learn more.
The History Press published my memoir In Service: The Story Of A Welsh Guardsman and on the same day of the launch hardback copies were available on Ebay. I queried this with the publisher and my agent at the time and discovered it was a loophople that had to do with the publisher selling damaged copies that I wouldn't earn a royalty on. A friend purchased a copy on Ebay and it was in perfect condition.
My point is we're surrounded by thieves, be they Amazon or Trad publishers.
Another story is about my first novel Raw Nerve. I self-pubbed and discovered shortly after pirate copies were available on a pirate site.
Is it really any worse than secondhand books shops selling your hard work without the author receiving a penny?
I don't know. I'm just going to keep writing because it is what I love to do.
At the moment I'm focused on scripts, but they're easily stolen too.....