Those signs are all over Cali. It's "The Boy Who Cried Wolf" on a massive scale. Nobody pays attention to it because it is on everything.

From Wiki:

The following warning language is standard on products sold in California if they contain chemicals on the Proposition 65 list and the amount of exposure caused by the product is not within defined safety limits.
WARNING: This product contains chemicals known to the State of California to cause cancer and birth defects or other reproductive harm.
The wording can be changed as necessary, so long as it communicates that the chemical in question is known to the state to cause cancer, or birth defects or other reproductive harm. For exposures from other sources, such as car exhaust in a parking garage, a standard sign might read: "This area contains chemicals known to the State of California to cause cancer, or birth defects or other reproductive harm" .

California Proposition 65 Warning
Some businesses in the state post similar notices on their premises, even when they have not evaluated the actual level of risk from a listed chemical they know is present. Warning signs are often posted at gas stations, hardware suppliers, grocery stores, drug stores, medical facilities, and many other businesses. Government agencies, parking garages, hotels, apartment complexes, retail stores, banks, and restaurants also post warning signs because of the possibility of hazardous chemicals being present in everyday items or the nearby environment. Some large businesses, such as utility companies, mail a Prop 65 notice to all customers each year to warn them of dangerous substances like natural gas or the sand used in sandblasting.

There is no penalty for posting an unnecessary warning sign. Because of the overuse of the vague warning, the ubiquitous signs ultimately communicate very little information to the end user. This problem has been recognized by California courts, advocates, and businesses.