One of those restrictions comes down to what members of the royal family can eat in public. We already know that the Queen has a very specific diet, but there’s one food that is forbidden, no matter who you are: shellfish.
According to the Sun, the Royals are not allowed to eat crustacean cuisine while eating out, which explains the lack of photos of Prince William and Kate eating at Red Lobster. The rationale behind the rule isn’t based in some archaic slight against the house of Windsor by a 20th-century fisherman, but as a precaution against illness.
Filter feeding shellfish “can concentrate harmful bacteria and viruses” and lead to “ gastroenteritis, septicemia, salmonellosis, hepatitis, cholera, and typhoid” as well as just regular old food poisoning, so this precaution is very understandable.
A keyword here is “public.” So, hypothetically speaking, behind closed doors and away from the cameras, Harry, William, the Queen and the whole royal family tree could be going to town on a ziggurat of king crab legs right now.