<i>Getting drunk IS a religious obligation on Purim. Sure, some may hold of other shittos, but secular law allows the freedom of religion to abide by any shitta. The law cannot make a distinction and force one shitta over another. Indeed it would be unconstitutional for the law to to decide a religious matter.</i>
You might want to look up the court case “Reynolds V United States” (1878) The court held that despite the first amendment the government could restrict acts that may have bearing on religious practice. In that case it was polygamy but I suspect that giving large quantities of drink to a minor would fall under that precedent.
If you are going to drink on Purim please be safe about it!