Not exactly illegal, but it's considering cheating, and due to past experiences with hacking and such in previous Animal Crossing games, they wouldn't want anybody getting ideas, I guess?
I don't think there is really a "gag order" on talking about hacking on this site. You just cannot exchange hacked / duplicated items on this site and you also cannot post threads asking people to duplicate items with you. There are plenty of hacking / duplicating discussion threads on this site which have not been locked, so you can talk about it as long as you aren't trying to use it to trade on this site.