I am actually very interested in this question. A 51% attack *should* be illegal. Although I think actual laws addressing 51% attacks specifically are a long long ways off. In a lot of ways governments have an incentive to never make laws that would improve the stability of bitcoin or crypto coins. If there were laws, especially international laws that made 51% attacks illegal the price of cryptos would be much less volatile. Although the laws would not prevent attacks completely, it would signal to the average person that the crypto world is a controlled and monitored market. Government may never accept crypto and protect its very specific needs.