Why do the music services vary in selection?

This is an oddity. Napster and Walmart both have Nickelback songs online, including their current “Someday”. Walmart also has Linkin Park songs, but not “Numb” (or anything else from Meteora). Since it appears that Linkin Park has agreed to single song downloads, why don’t Napster and Walmart have everything by them? Is it because they want to sell as many traditional CDs as they can before releasing for single-song downloads?

And, if Napster and Walmart both have Nickelback and Linkin Park, why doesn’t iTunes have either one? Apple has deals with all five major labels and is far and away the current leader in the music downloading biz. Why wouldn’t you find those songs on the leading service?