There is no such rule. It's more like, "it's illegal and very, very bad to upload copyrighted material without permission... but if you happen to do so despite our stern warning, we'll look the other way, because the copyright owners make money by running ads on your video, and we get a cut of that revenue, too."
There are some artists who are steadfastly against allowing their music to be posted online, and will file copyright takedowns against anyone they catch doing it. Roger Hodgson of Supertramp and Don Henley of the Eagles, for example. But they are getting fewer and fewer, as they either die off, or sell the rights to their music.