I think it is pretty obvious. While Jack London's martin Eden is out of copyright and public domain now, if they order Penguin edition they still have to pay them some money for it. So Amazon calculated that it is cheaper to print their own. My guess they could not do this for non-public-domain books without securing rights first.

But I don’t think Amazon could “pirate” a Penguin edition like that without their consent. Russell’s The Problems of Philosophy is like you say, but they stripped it out of all mentions of a publishing house. I didn’t mind that one in fact