I know this is an old thread but for unrelated reasons, been thinking about this too. My answer - or my hypothesis anyway - is that 'dietary' (human diet) carbohydrates (sugars, pastas, some breads, etc) are mostly being lumped in as 'greens' even when they don't fit the strict C:N framework yard waste is generally looked at in. And I think that's because these starches and sugars are so easily digested that they cause a burst of microbial activity (who mostly bring their own nitrogen i.e. proteins to the party and reproduce like mad, get eaten, return their proteins to the pile as urea, etc). So even if pouring straight sugar onto your compost pile doesn't directly add any nitrogen, it acts more like a green. And if you look at the ways compost manuals talk about the C, it's focussed much more on the cellulose and lignin. The carbs we eat just behave differently (I think part of it is their relative solubility but out of my area of expertise). Or in a dietary comparison: the 'starches' are not the relevant C, but the dietary fibre - the parts human digestive systems mostly can't directly break down, that's the analogue to the 'compost C.' Let's call it compost fibre, since it's not so much the elemental C that impacts how different materials act in a normal compost pile. Caveats: some foods like breads do have more protein in them than we tend to think. It'll depend on what the 'biodegradable things' are actually made of. Other part of this hypothesis might be that because more soluble, they don't keep any structure in pile and act like a lot of the wet greens too, most of which break down faster.

Also, aside from being utterly non-organic (this is me saying they are manufactured from petroleum products, not that they don't contain carbon ;). Vermiculite is a natural mineral, altered in physical structure by heat, but otherwise perfectly normal... I'm not uncomfortable about having it, in small, loose particle form, around things I am going to eat, (it is a major component of the soil and rocks where I grew up) I can't say that about Styrofoam. Aside from being nearly worthless for the intended purpose of vermiculite, they will rise and float to the top in a way that will put even perlite to shame.

they're ok, but they dissolve in water so will disappear quite quickly into the wet bedding .... I add large quantities of them to my compost bins regularly Bill

When I get styrofoam peanuts I just take them to the UPS store (any packaging store) they are more than happy to recycle them. Just ask if they can recycle them and I'll guarantee you they'll say "yes"!