In Peaceful mode, Hostile mobs are recognised by commands when being summoned by a player but not by a command block. So, for instance, this command is placed in a command block (any):
/summon Ghast ~ ~ ~
When activated by a player, the Ghast spawns, despawns, but is also detected by other commands running.
(So, '/execute @e[type=Ghast] ~ ~ ~ /say Hi' is successful.)
And when activated by a command block, the Ghast is summoned and automatically despawns, without being detected by the same command aforementioned.
Perhaps they're not meant to be detected in Peaceful mode at all, or perhaps (and I think that) they should be. Either way it's inconsistent.