Why I am not a vegan

[Unbound]

That doesn't make it a false premise but that can be accounted for by evolutionary adaptation. Just because caterpillars have developed a structure that is accepting of mustard oils in its digestion doesn't mean it isn't still a defense mechanism nor does it prove that it is therefore acceptable for humans or any other creature to eat it. Maybe the caterpillar and the plant are in symbiosis, but that doesn't mean that relationship is the same with every other creature or even that it is necessarily desirable to the plant. Maybe the caterpillars are a parasite or predator for the plant and they have developed a way past the plant's defense mechanism? That is usually how predators work and why predators are dangerous to their prey.