I have eaten prawn sandwiches. They reson your not ment to eat shellfish is due to the risk of food poisening.
Personnaly I thank provided your are carefull about the seafood you eat, you shouldnt have a problem. If you properly defrost prawns and eat them straight away and aviod getting shellfish from anywhere where they have not been refridgerated right up untill consumption, this should not be a problem. I think the guidlines just say no to all to as to aviod any confusion about what is and isnt safe