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.
I think that the main focus in this situation should be done on the technical aspect. The legal side is clear. It's illegal because the ultimate purpose of this action is to take someone else's property. It seems to me that 51% attack can become a global problem for all coins that use GPU mining.