Because they don't mean anything and they only serve to desensitise people to warning labels.
When I get a power strip at a corner store and it has a big label which says "WARNING: This product contains chemicals known to the State of California to cause cancer and birth defects or other reproductive harm." I can only roll my eyes and marvel at the stupidity.
I hope these people won't hear about the dangerous chemical called dihydrogen monoxide...
Got it. Do you know of a sane set of guidelines that I can reference?
Press Release
Edit: Soylent's Reply, provided by Trevor_Blake