I am the editor here, and no one is doubting your story. They are just trying to figure out some of the details. You can gripe here all you want.
The bottom line is that food poisoning can happen anywhere, and it may or may not have been the cruise line's fault. Mosty cruise lines are VERY careful about the food because they do not want an outbreak of anything.
My wife got horrible food poisoning in the Atlanta airport once - she was so sick we needed a wheelchair to get her off the plane in London. Another woman was in the same condition, and we figured out they had both eaten the same salad at a particlular "natural foods" restaurant in the airport.
They were lucky to have figured it out. In your son's case the doctor said it appeared to be food poisoning, but we don't know where he got it. When you cannot specifically pinpoint where it came from you can only assume it was RCL's fault. Other people would have had it as well. You said you knew exactly where he ate - and the doctor did not appear to know of any other cases, so that is why people are just pondering.
If you know the sickness was their fault you can write a letter, and it may help, but I think most of us have our doubts, just being honest. It is like if you leave your pocketbook on the subway - who knows what really happened? There isn't always someone to blame.