There is a ton of realyl silly stuff in manuals indeed, but it is there because there are people stupid ennough to do that, get injured and then sue the company for damages because the manual didnt state that this would be a bad idea. Some of those silly lawsuits have been won by those idiots so companies try to cover all possibilities to avoid liability.
My personal favorites are chainsaw manuals where they tell you not to stop a working chainsaw with any part of your body
