Another orally handed down 'law', made enforceable by peer pressure.
I remember that it was 'looked down on' to swear on the Bible. JWs, being a very black and white crowd, were told that 'christians' didn't need to swear on a Bible, and most took it to heart, all the way to refusing to swear on a Bible in court proceedings. Even when the WTS clearly called it a 'conscience matter', JWs were still afraid to go too far.
That's where these court proceedings can be tricky. If an attorney starts harping on these minor little things, and then a WT lawyer trots out printed material that leaves it up to the follower, then it can make the accusing lawyer look at best silly, at worst misinformed.
We know that real WT law is an oral tradition, and that words in a Watchtower are just a CYA for later, should someone accuse the WTS of something. Problem is that you can't easily prove this indoctrination method straight up. You have to experience it. You can't really prove it, unless you lived it.