From Tim B:
"Realistically, the spiders probably shouldn't be immune to poisons either... What's the first thing 90% of people reach for when they find a big, black, hairy spider in their loungeroom? A can of - you guessed it - poison."
While it can make some sense for Cave Spiders, the normal Spiders are immume to poison. And unlike Cave Spiders, they do NOT use poison and they not have it. Making them act normally for poison can make sense for players.
While Cave Spiders, however, are likely immume to poison due to them using poison of the type from the Poison effect.