I didn't see the site, but at a guess, HarryT checked the copyright dates of several books on the site, and made the decision based on that evidence. We do try to be sure something is a copyright violation before editing posts and such. And we have to take into account the laws in the countries our servers are in (the US and Canada), whereas we understand copyright conventions can vary in different countries.
Unfortunately, there are many sites out there that assume that because content appears somewhere it must be ok to post it, or think that something can be "Creative Commons" if someone other than the author has said so.
We're left in the awkward position of trying to figure out what the actual legal status of books is, sometimes.