Blame George Carlin, rot. I think his rules for TV apply here...
I am not saying that poop isn't what goes in the worms gut, but the majority of his nutrition comes from the biology residing there, not so much that actual humus and content. The majority of THAT is expelled as castings (the worm has added mucilage and some other goodies, and ground it down more, but the bulk content is mostly still there).
It's much the same with us but we tend to start with a less refined and more complete product, so our chewing and digestion allows for a more complete chemical extraction of the nutrient. We are almost done with what we pass along; the worm must make do with what's left. Some very depleted humus, with some biology along for the ride.
Bacteria and fungi are far more happy with the foodstuffs we pass along as they are much further down the food chain and are really the ones cleaning things up, then the protozoa, nematodes, microarthropods and THEN our higher level predator, worms. The worms are more eating everyone preceeding them in this paragraph than anything else; the substrate is just the medium that they chew. Think of it as roughage for worms...
The spinach scare was pretty well linked to a nearby CAFO; the surface water flow went from there to water supplies for the irrigation. Poor agricultural practices that lead to surface compaction (increasing surface flow) and high-intensity feeding (leading to hardier, more virulent strains of E. coli) are issues beyond the scope of this discussion...
And I kinda like Latin. If we are gonna talk dead decomposing things, a dead decomposing language seems just about right...
HG