[Quoth]

Sometimes shorts have been in USA magazines, that as part of deal they got the copyright and back then USA magazine copyright had a shorter term, though maybe there was a renewal option.

That seems to be the case on USA Gutenberg, where certainly the novels are not there because the author isn't sufficiently dead.

Edit:
Also big mainstream publishers also publish work that is certainly PD, if they think there is a demand.

Amazon only cares if a proven rights holder complains via lawyer. They ignore 3rd party complaints about IP (for anything) for obvious reasons, including not getting sued by the seller. They mostly ignore safety issue reports from the "public" too.