There is illegal stuff in there, because some of the books they've swept up into the archive have been retconned to be illegal. A lot of obscene material wasn't illegal until into the 80s, and these shadow libraries are scooping up everything without checking. I don't know what they do if you report it to them.

For instance, there are issues of multiple issues of Playboy with underage models. All the archives of British tabloid newspapers had to be purged in 2003 after the laws changed there, etc.