They don't want to admit that they need these videos to promote their artists for them, but because of the company's policies, and copyright laws they will eventually delete everything, but as you may notice, they don't delete everything at once, just little by little.
Just notice how some of her lives have been uploaded since 2009 for example, and they're easy to find.
I haven't uploaded videos to youtube in ages, but the last time I did they got deleted because I used ayu pictures and music
I hope things are different now.
What kinix said sounds pretty interesting, I guess that's a way to protect both the person who uploads and the copyright's owner? Perhaps it's up to the copyright's owner if they want to allow those people to keep their videos, as long as they say who owns the material.