Someone who sells plastic rice as the real thing should be charged with a capital offense. First and foremost the absolute lack of nutritional value will violate whatever is on the packaging and be the first line of harm. But more than likely this product could kill not just from malnutrition but from actively sickening individuals.
Note that Snopes has posted about this rumor and it's been debunked. However if someone actually sells plastic rice as the real thing, then that person/entity is criminally liable. Eating plastic rice is potentially harmful, and if an unaware consumer ingests it, he or she could get sick. Defrauding people is a crime, as is harming them.
Misrepresentation of a product is bad enough, but selling plastic as a food product should definitely warrant worse punishment. A misdemeanor or fine would likely be like a slap on the wrist to some people. However, this lie sounds like it has potential to be deadly or at least seriously detrimental to someone's health.
While this scenario would be a serious crime, capital punishment is usually only given in cases of premeditated murder. It would have to be proven that the rice was used as a murder weapon. The health effects of consuming plastic rice are not known. It would also have to be proven that the seller of the rice knew that it was plastic.