"I'd argue that pirating games that aren't available anywhere should not be illegal. "
The only reason that I can see that this would be illegal would be due to copyrights.
I don't think they're arguing that the original author/publisher shouldn't recieve attribution and credit for the original work.
If someone was originally selling a product openly, and they now no longer choose to do so - then they're effectively abandoning their interest in those copyrights. Sharing those abandoned items shouldn't be illegal.