Before we do any laughing at others, let's find out the actual jurisdiction involved. Europe has a lot of curious sexually-related prosecutions too.
The keyword might be "sharing", meaning you are sending out something "illegal", and what's "illegal"? Is it about copyright infringement (the new Avenger movie), or about sexual mores (despite the mosaic, many JAV violates some local morality laws pretty much the whole world over).
Check your torrent client maybe you want to reduce your share ratio.
Also don't get too worked up over an unfriendly email from your ISP (instead of your friendly Department of Justice or FBI). Most likely you have a choice of ISP's in your area. If your ISP is being a prick, just take your business elsewhere.
There's a very good chance that the threatening word (illegal) is just a veil to cover the ISP's true motive of profit making. They want most of their customers to pay for high speed internet but not use much bandwidth.
Anyway you are hardly downloading anything from AO. AO hosts a forum and only a tiny amount of media, we are mainly just leeching DD links and torrents.
And I doubt the JAV studios truly don't care. They just lack the means to enforce their claims.