Some people prefer the “feel” of a thin layer of polyfoam vs the “feel” of sleeping directly on latex which is a much more resilient material. Others prefer a natural fiber quilting and some prefer sleeping directly on the latex. These are all personal preference. There are many manufacturers that use a thin layer of polyfoam in their quilting for this reason among the others I mentioned earlier. I don’t think they’re discounting the fact that there are some that prefer sleeping directly on the latex as much as making what they believe more people would prefer.
Soy foam is just another name for polyfoam that has replaced a small percentage of one of the two main petrochemicals used to make it (the polyol) with a polyol that is derived from soy oil. you can read more about so called “plant based” foams in post #2 here.
Several of the online manufacturers on the list either carry or can make a mattress with a stretch knit unquilted cover including …