I doubt a lot of the 15-18 year old Avenged Sevenfold fans have listened to Cannibal corpse. They might know who they are, and know that they're a death metal band, but I doubt many of the fans of the mainstage bands are that familiar with them, so it's a chance to expose them to a new audience. You also make more new fans when doing a support tour than you do on your own headlining tour. And Cannibal Corpse has more in common with Dark Funeral and 1349 musically than they do with Avenged Sevenfold.
08/08 - Slipknot/Lamb of God
09/30 - Revocation
10/04 - The Black Dahlia Murder
11/12 - Kataklysm