The legal system exists to temper the interplay between wealth and power -- instead of a completely unregulated environment where someone can buy up mercenaries

The legal system exists to temper the interplay between wealth and power -- instead of a completely unregulated environment where someone can buy up mercenaries and become their own state, we have reasonable market regulations that keep the playing field fair for all comers and prevent the super-rich from affecting unfair distortions of either the market or society at large. If seriously corrupting effects can occur from one person's use of money, we have a larger issue and need to address the cause (someone can use money to cause a big, society-wide problem), not the symptom (someone has a lot of money and is using it in a socially-disastrous way). If we're nitpicking at individual's monetary use, the system is broken. Individuals should be able to amass any amount of money and not cause large-scale social damage. Looking at a set of fortunes above a certain size and confiscating them because we can't build a resilient system is totally bad. In effect, we're already doing it with income tax. If you don't contend that Zuck will cause large-scale social damage, what's wrong with him having that much money and using it as he sees fit? After all, he "earned" it.