Blame the gov't first for that: they took someone's property for non-payment of "protection money" (aka "taxes"). This guy just did what none of the other homeowners bothered to do: buy the property, either from the prior owner or from the gov't. He didn't steal the property, the gov't did.
He may find that he doesn't own the lake either since it is, by law, part of the "waters of the United States" and therefore subject to the wetlands regulations.
He may not own the water space, but he does own the 10-20 foot strip around it.