Yes, it does suppose that.
That people display the warning without specific knowledge of a hazardous substance is the fault of the California legislature for passing laws that provide perverse incentives to do that.
If a business could be held liable for posting the warning without a reason why, those warnings would not appear without reason. Ambulance-chaser attorneys perform a kind of arbitrage there, as private-party enforcers, like finding ADA violations at a non-accessible business and suing to force them to settle and make reasonable accommodations, or fight it, pay fines, and make reasonable accommodations. Or like the Florida folks that test their Sunshine Law against various municipal organizations. The munis are forced to settle and open their records, or fight it, pay fines, and open their records.
I guarantee there would be at least one person in California that would go around checking for warnings and asking for the MSDS reports on the hazardous materials on the property. I asked for a MSDS once, and the building manager just about lost his mind. I just thought I smelled toluene, and wanted to check to see if I was right. Their reaction made me think that maybe someone should have been suing someone, and they were just worried that it was going to be me.
As with any complex system, it's the squeaky wheels that get greased. More sensible laws would make California less an object of ridicule for other states. Indiana takes a lot of flak over the pi thing, and Kentucky and Tennessee occasionally get their "ice cream cone in the back pocket" laws waved around, but those are just silly artifacts of law. The California "causes cancer warning" law affects so many businesses, even those outside the state.