Let us not focus on the technical aspect or how 51% attack is done. The attack is like double-spending and what have other have called this attack it is equivalent to a digital counterfeit. And we all know a counterfeit currency is illegal in any country for some it is even considered as a national threat. Double-spending itself is fraudalent by nature meaning it is punishable by penal laws. They are basically taking advantage of someone just for their gain.