It's worth noting that in Japan, copyright violation is a criminal offense rather than a civil offense and can constitute up to 10 years in prison.[1] Even downloading copyrighted materials is a criminal offense there.[2]
Suing doujinshi artists (or cosplayers, for that matter) for promoting the copyright holders' materials would be in bad taste and would likely drive fans away from the copyrighted material. Considering the reputational consequences and low "reasonable royalty" damages from suing individuals, copyright holders don't have much incentives to shut these conventions down.[1]
Copyright holders might not, but isn't one of the main distinctions between civil and criminal law that, in the latter case, the state can (and in theory is obliged to) prosecute criminal cases regardless of the copyright holder's thoughts?
In Japan, copyright law can't be enforced without the complaint of the rights holder.[1] If you have some spare time, I'd recommend reading the Copyright Law [2] for the powers that are legally provided to the rights holders and specifically, "Measures Against Infringement" [3]. They're quite fascinating.
Disclaimer: IANAL, much less a Japanese legal scholar.
Some IP holders do sue. Banpresto sued a group making parody footage of their flagship game because the group was immensely popular and was making $x00,000 per event.
Basically, as long as you aren't overly successful, they won't come after you.
I'll have to ask him again. He's a bit tech illiterate, so that might be it. Plus, being there as an ESL teacher you don't want to jeopardize your career with anyone finding out you torrent.