Copyright infringement is normally related to selling or providing copyrighted content. The person that's uploading the video is the one violating copyright.
Although the law is a bit more then that, that's basically what is happening.
and no, about 90% of the audio tracks/videos on youtube are violations of someone's copyright.
As an individual you shouldn't worry too much about copyright unless you're selling or uploading the content.
Edit and Disclaimer: Just because they don't "care" for you downloading songs without paying for them it doesn't make it right. if you enjoy a author's work by all means, buy the sound track from a place such as iTunes.
Edited by NpaMA, 07 December 2010 - 12:56 AM.