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.