The reason they need licence-free music is because they know that the use of unauthorised music in their videos could lead to their video being pulled by Youtube for copyright infringement. What's more, they could be laying themselves open to a legal challenge from the music's owners.
It seems that it is not generally known that Youtube have a library of copyright-free music and sound effects that are available for free download by anyone with a Youtube account. These downloads are all absolutely safe to use in all Youtube videos (and indeed, any other videos for that matter).
So this video shows you exacly how to go about accessing these great freebies - Courtesy of Youtube. Check it out. You'll be glad you did.