Royal Norfolk Greenbriar International Inc. cups

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I bought many! of these cups at a dollar store to give as Christmas gifts. They have attractive artwork on the outside-- and are actually more a "mug" than a cup. I was extremembly disappointed when I took one out of the box only to have a "warning" pasted on every cup to the affect that the product contains chemicals known to the State of California to cause cancer and birth defects or other reproductive harm. What is in these cups which make them carcingenic? Do you have any information on this? Why are they allowed to be on the market?