We demand a worthy reward for defeating Commander Zhalk!
I'm fairly sure they didn't give XP or fun loot for these two because they essentially fight each other whilst we watch on, and if they die it's because we nautiloid nuked them or they just beat each other into the ground and we plinked off their last HP. Killing the 2 later cambions does give XP though, and that is actually pretty valuable because it let's you get Level 2 in the tutorial and Level 3 before the first Goblin fight.