FYI....I AM NOT BIOLOGIST.....
I believe the answer lies somewhere in the difference between the metobolic rate of fish vs inverts. Fish exert and consume much more energy then inverts, which tend to be slower moving and in the case or corals, non-moving at all. This increased energy consumsion leads to greater waste in the water.
A second factor would be corals and annenome get a portion of their energy from photosysnthis, which I believe produced oxgen as waste. I don't think we are concerned about excess oxygen in our systems.
I am sure that you could pack a tank full of snails, shrimp, seastars crabs etc and "exceede" the bioload of their environment, but in the small populations that we are dealing with, I do not think that it is a problem.
Lastly, you have to look at your tank as a system, and not a tank. The 1" of fish per 6gal is a general giudeline. Constant monitoring of you Ammonia, Nitrite and Nitrate are better indications of how your system is doing and if it is in balance or not.....
If I am full of BS and someone can offer a more scientific answer. then I appologize in advance and I wuol dlike to know what I got wrong........