Well to be fair I think not before codex deamons and the avatar there was no need for the deamon rule to be there so the gear worked, but when those 2 books came out things changed and the things called deamons needed the rule to also be affected.Without a faq the stupidity will continue im afraid.
Daemons were a major component of the Chaos codex in 3rd/4th editions. They were split of as a seperate codex at the end of 4th.
So, history lesson here. There was a Chaos codex, and it was good and awesome, and had daemons and spiky marines. You could make either a Daemons army or a Marine army, or a mix with this book. Then, there came along a book of Daemon Hunters, who had special rules against Daemons. There was no "Daemons" rule, but people were sensible, and knew a daemon when they saw one.
This golden age went on for many years. Eventually, the Eldar 4th Ed codex came out, in which was the Avatar. It had a "Daemon" rule for the avatar. The reason was obviously because a lot of people wouldn't realise it was demonic, if not specifically told about it. That was all fine, there were daemons in the chaos book, and the Avatar. Everyone knew what a daemon was, and there wasn't even a rule for it.
Then, tragedy struck, the 4th Ed chaos codex was released. They still had daemons, not interesting ones however. It might be noted though that, just like in the previous codex, there wasn't a "daemons" special rule in the book. People still used Daemon hunters, and the Daemons in the Chaos Codex were still affected by them. Everyone knew this, and it was obvious.
Finally, C:CD came out. A whole army of deamons, and the designers decided to bunch up all their special rules into one happy package - the Deamon special rule, which combined EW, fearless, demonic assault, invulnerable and demonic rivalry into a single, catch all rule, so they didn't have to repeat them all for every entry in the book. This is where it gets complicated. For some reason, people suddenly don't know what a "Daemon" is anymore. Even when the Avatar had the "Daemon" rule, you'd get laughed at for saying it was the only daemon affected. For 5 years, people could tell you what a daemon was, but now it's debatable. Because there's a Daemon rule, it doesn't mean we shouldn't treat those other Daemons just as we have ever since the DH codex came out. Nothing else changed, the designers just used a catch all "Daemon" rule to help cut down on the number of rules they had to write in each entry.