> Meanwhile if the stuff is actually dangerous then it's dangerous in all 50 states, so the warning label should either be everywhere or nowhere according to the evidence, right?

Only if other states or the federal government give that much of a shit about food safety, which is not a guarantee, both in theory and in practice. They might, for instance, care more about agri-profits than California does.

> So they could preempt California from requiring labeling on products shipped from Ohio, or require specific federal labeling on the things that are, but only California gets to decide about the things that never leave California.

That's just a regulatory-arbitrage race to the bottom. You'd just have out-of-state producers that don't have to follow any of your laws out-competing local ones.

It's the opposite of a race to the bottom. The federal government sets a single standard for the country. The same logic you're advocating is also the conservative argument for health care regulation --- that is, allow the states to preempt the federal standards so they can offer cheaper insurance by lowering standards.