I was fighting mobs on creative, and noticed some of them didn't fight back. Those mobs are: Creeper, Skeleton, Zombie, Witch, Silverfish, Slime, Ghasts, and Magma Cubes. They worked fine on survival though.
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Duplicate of MC-15111, please use the search function to see if your bug has already been submitted. Currently over 58% of tickets are being closed as duplicate.
I'm pretty sure that's a feature.