I'm sure everyone can tell what I was craving when I wrote this update. Based on the comments, I felt some adjustments and additions were warranted and to be significant enough to be put in the update proper rather than just in a response. So the below has been added to the original update:
The rules were fairly simple. Pizza was never to be consumed alone, as it is often consumed in modern western countries. It was to be accompanied by a salad or fruit or both. But these vegetables and fruit were side dishes, and could only be side dishes. Fruit or vegetables absolutely could not be on the pizza itself, the only exceptions being the contents and bases of the sauce, cheese, and any seasonings. In this range there was much variation with types of cheeses, sauces, and seasonings, but in terms of toppings as one would order in a restaurant, only types of meat were allowed. (There was some regional variation in what counted as meat, specifically types of seafood and eggs, for this purpose. Cyprus, Crete, and the Pontic Coast also included mushrooms, onions, and later bell peppers as ‘meat’ for this purpose, which the rest of the Roman world simply took as unnecessary confirmation that those Romans were weird.)
It should further be noted that for many Roman pizza-consumers at this time, this was largely a non-issue given the expense of meat. Many pizza-consumers were eating what would today be considered a simple cheese pizza with no additional toppings, although they experimented with seasonings and sauces. However this factor also helps explain the strength of the pizza etiquette at this time. For the vast majority of Romans, eating pizza with meat toppings would be a special event, possibly marking a significant event and done in public. In that context, keeping etiquette, avoiding shame, and avenging insults was especially important. (This also explains the relative weakening in later times, when eating meat is more commonplace, and thus pizza becomes less of a special event and more likely to be something to be eaten on a meet with friends.)
How or why these specific rules arose, as well as their practically-universal adoption, is a mystery. It does seem to have originated in Constantinople and then exported to the provinces. Some believe its origins lie in nothing else than the personal preferences of Demetrios Sideros, or perhaps that of Empress Jahzara. There is an apocryphal story of a cook trying to serve her pizza with pineapple on it, and her responding with ordering the removal of his tongue. That was either an extreme overreaction, or a sign that she was a champion of justice. Opinions differ on that to this day.
In this possible explanation, the meat-only pizza was simply a personal taste of the Imperial family, which was then copied by senior officials, and then by their juniors first in the capital and then in the provinces. The social rationale for the customs came later, as a way of explaining this seemingly random pickiness.
