In any other genre besides rap, this is a very common thing. Very rarely does the singer of a song actually write the material, especially a new act. I.E. when Britney Spears first cam onto the scene, those songs were all written by other songwriters. After she got famous and people would buy anything she put out, the label actually allowed her to write some of her own songs (with help, I would assume). You always hear big names singers (especially in pop music) say in interviews right before their new album is about to drop that they actually wrote a few songs on the disc. Most likely those said songs are not the biggest draws (singles) because the majority of pop artists are in it because they can sing. And they'll sing whatever the label provides them with.
In hip-hop it is also very common, but more so for producers trying to rap, such as like P-Diddy (or whatever he calls himself now), Dr. Dre and so forth. With Dre you can kind of tell what artists he's working with at the time by his own lyrics...why? Because they are the ones writing his material. During the original Chronic record, it was Snoop that was writing a lot of his material, and for the second one, probably Eminem. I know Royce Da 5'9" did "The Message" off of the chronic. If you look in the liner notes, you can usually tell by seeing who the publishing belongs to. There were a handful of songs off the latter Chronic (2001) album that had publishing go to Eminem even though he didn't apprear on the track.
As for Kanye ghostwriting for Jay-Z??? I just can't see it. Those two are like apples and oranges. But if it's true, it's true. I don't know.
Hope this helps.
-Springfield