The Bug

The gamerule "doMobSpawning" is supposed to affect mobs (of all types - ex: passive, hostile, etc.) that are naturally spawned (which excludes things like mob spawners). However, setting the gamerule "doMobSpawning" to false does not prevent passive mob spawning (at least in newly generated chunks - I haven't been able to accurately test if passive mobs will spawn outside of generating with new chunks).

Steps to reproduce:

1) Create a new world
2) Run the command:

 /gamerule doMobSpawning false 

3) Move outside of pre-generated chunks
4) Observe passive mobs such as Chickens, Pigs, Cows, etc. being placed in new chunks as they generate.