From what I recall at the time the issue with back of house workers is that if anyone on the line has it then everyone is likely to get it. The kitchen is crowded, it is hot (making PPE seldom used), people are shouting over each other and the noises of cooking, ventilation is pulling from the public in the front of house across the line and then out, and the existing contamination-prevention efforts are all based on touch and not respiration. While front of house people would have been exposed to the general public they were not as confined.