At this point, there should probably just be a generic "THIS PRODUCT IS KNOWN TO THE STATE OF CALIFORNIA TO CAUSE CANCER" sticker that companies can stick on pretty much anything they sell there. <eyeroll>
Feces are produced by the human body, too. Should you be eating them?
Just because something is also produced by the human body or is "natural" doesn't mean it is good for humans to ingest, especially in significant quantities over long periods of time.
I get the sarcasm, but would this be legal? It's satire if it's not actually known as a carcinogen, and a legal requirement if it is. It would dilute the warning in the public's perception, but companies might find it cheapest and easiest to just stick it on everything.
If something is carcinogenic is always a matter of volume. Most things are carcinogenic if consumed in quantities large enough. Most things carcinogenic if aerosolized and inhaled. Most things are carcinogenic if left on your skin long enough.
The latest is maple syrup it now has to have a warning it "may contain lead".
Which is I think is an inflammatory way to phrase it. There should be a level shown: "This product has lead levels of 11ppb, California drinking water has lead levels of 11ppb".
edit: Oh and apple seeds naturally contain cyanide, bananas have radioactive potassium-40, fish has mercury and on and on.
WARNING: This human being contains chemicals known to the State of California to cause cancer and birth defects or other reproductive harm.