digestion
I don't buy that eating fruit after you've eaten meat will cause it to stay in your stomach and rot. The pH in your stomach is so low and there are so many enzymes that it gets broken down way before it can "rot". Things such as meat may take longer to break down, because of their density, but your stomach churns stuff up a lot, so the order you eat is not entirely relevant.
A lot of the traditions of when to eat a salad or sorbet in respect to meat or the rest of the meal have to do with cleansing the palate and preparing it for new tastes. Salad and fruit are good for cleaning your taste-buds essentially, which is why its often served between courses in Europe, instead of at the begining like in the US.