It does that because it thinks that you stole the song usually, like when you sound record it. Thats called 'third party'. If you didn't buy the song from itunes or another store or have the CD, then it will think you didn' t pay for it somewhere, therefor don't have the right to upload a vid with it in it.
. I would say upload the song with it from itunes, or another song buying site. I'm not familiar with a lot of stuff like that, but have had several vids with the same stupid song copyright problem.
Hate youtubes copyright..