The problem with Amanita muscaria is that it contains ibotenic acid. This causes irreversible brain damage but is metabolized in the body into muscimol, which like the other alkaloids in the mushroom, is a heavy-duty hallucinogen but with no long-term physical effects. So the person who eats the mushroom itself is risking a lot more than hangovers, while the pee drinkers just get safely smashed.