What’s the rationale there?

maybe to keep nutrient content of natural products high?

If you start to fortify everthing, food producers will just put the cheapest stuff inside. Moreover they will not care if their processing method destroys micronutrients.

I think this type of intervention recognises that an element of pragmatism can have a significant impact on population scale healthcare for a very modest cost and with low risk. People who get to worry about how long their wheat has spent touching stone aren't generally the ones who the intervention is targeting. They're different issues at different scales.

For another example, see the impressive work of Iodine Global Network - https://ign.org

Is there some corresponding law that prevents the cheap, no-nutrition food from being sold anyway?

Probably the same idea as the one that is often claimed to be behind the Reinheitsgebot from 16th century Germany (Bavaria?) that regulated the making of beer, that is, to ensure purity.