That's not the best example, since the levels set for Prop 65 warnings are so low that the warnings are effectively useless; every single commercial building in CA now somehow causes cancer.

Surely we both understand the point I was making in that labels are already compelled by laws today.

Fine, cigarettes must be labelled as being a risk of causing cancer. The punishment for failing to do this is both civil and federal penalties including massive fines and federal prison time.