Why cook green plantains?
According to the FAO (Food and Agriculture Organization of the UN) "Banana and plantain do not contain significant levels of any toxic principles."  Raw, unripe plantains can be difficult to digest, especially if eaten in large quantities.  Doing so can lead to upset stomach.  This may be due to the fact that they contain starches resistant (RS2) to digestive enzymes, compounds that inhibit salivary amylase production, and significant quantities of amylose. 

Raw, unripe plantains and bananas also have a starchy or waxy texture and a bitter flavor which can be unpalatable.