Why do only some mobs attack in Creative?

Question

Zombies and Skeletons will remain passive in Creative, even if I hit them.

Spiders and Endermen do become hostile when hit.

Why the discrepancy? Is this explained anywhere?

Answer 1

The behaviour of mobs toward Creative players was only recently changed. They used to agro when attacked, but ignore you otherwise. They would "forget" about you if you went out of their vision radius (16m).

The recent change seems to have removed the agro effect caused by attacking some mobs. The mobs that no longer agro even if you attack them include zombies, skeletons, and creepers (having just tested this on 1.6.2). Spiders and endermen still agro when attacked. (Endermen do not agro when looked at in Creative mode; other mobs such as wolves, silverfish, and bosses not tested.)

What's notable is that both mobs that lack the new non-agro behaviour are mobs that have custom agro code. (Spiders agro differently during the day than the night; how endermen agro differently is well known.) Looking at this, I can infer that the discrepancy is very, very likely due to a change to the code that was made only to the common mob code. Presumably it will be changed in the custom mob code as well at some point.

To test this hypothesis, I went looking for zombie pigmen to harass. Since they have custom agro code, they should still attack when attacked. Indeed, when I bothered them they agroed as expected.