The sun's radiation causes cancer, yet placing a blanket warning stating the sun causes cancer likely leads many to wholesale avoid the sun, which would be folly (humans need some amount of sunlight).

Similarly, imagine if nutrition facts labels only had to state whether the food could be potentially unhealthy, without any other nuance.

A sensible version of Prop 65 would acknowledge the specific substance, its dosage, and the best known safe dosage. Equipped with that information a consumer can make an informed decision.