My maids fed my rabbits iceberg lettuce !!!

I have no idea what shall I do.
My rabbits are running in yards
Will it kill them.

They will be okay, it’s not poisonous. So long they eat normal and their droppings are okay. You have nothing to worry about.

Why is iceberg a no-no? We avoid it because it's on all the rabbit sites as a DO NOT FEED item. But if it's not poisonous, why must it be avoided?

Well, I would say that DO NOT FEED meme is kind of an exaggeration. Iceberg is pretty much devoid of anything useful, so it's not good rabbit food, definitiv no staple food. Giving it to rabbits that are not used to greens can cause problems like diarrhea, imho more easily than other green stuff that isn't basicly water. A rabbit used to greens shouldn't have any problem if not an excessive amount is eaten (those rabbits quite likely know anyway when they've had enough)

That bitter stuff that is cited to be toxic is in every other lettuce of this plant family too, and compared to their wild ancestor (very yummy rabbit food, in moderation) there is very little of it.

Iceberg lettuce can become harmful in large quantities, make their stomach get bad and so on.

Also iceberg lettuce have a high value of water which make them not so healthy for rabbits.