I've noticed they don't have ac/dc and some others. Is that just because they dont have permission to sell their music?
yeah the artists have to give permission and sign a contract saying that itunes will get an amount of the profit.
Some artists just choose not to be on the service. The Beatles weren't on iTunes for years until a few months ago.
As for AC/DC, their reason for not being on iTunes is that they don't want to sell their songs individually, and also to protect the whole album format.