That's simply not true. They have to list the nutrition info the same as any other food. Protein powders that are "mostly filler" are actually ~20% filler, not mostly. And the label makes that quite clear as it both lists the ingredients so you even know what they are using as filler, but also the nutrition info. If 30g of powder is only giving you 20g of protein, obviously there's 10g of something other than protein there.

Im not thinking about the FDA specifically here. I'm thinking about general wrongful injury (possibly death) lawsuits where someone becomes seriously ill from consuming nothing but their "magic soylentesque shake".

Marketing it as a food replacement would be very difficult because of subtle differences between people's bodies. I think they stick with supplements/protein powder because it reduces the possibility of wrongful death lawsuits (and similar) while still allowing leeway with federal regulations.