the FDA investigating the restaurant because it potentially had more than 0.4% (?) of human flesh content. Why any at all? At such a small amount it's impossible to detect, completely safe to consume, and would be well less than a single finger in literal tons of hamburger

Not to actually argue against your point (nor to conflate this cartoon scenario with real-life regulations), but 0.4% would be way more than just one human finger in literal tons of hamburger. 0.4% of one ton would be 8 pounds / ~4 kilograms. I don't know how many human fingers that is, but I'm certain it's significantly more than one.