Why regulate natural monopolies? Conventional economic theory points to the price-marginal cost margin and the ensuing deadweight loss. But this hypothesis does a poor job of explaining the way that regulators behave in practice. This paper proposes an alternative hypothesis: that natural monopoly regulation exists to protect the sunk investments made by consumers of the regulated firm. This hypothesis explains many of the practices of regulators which make little or no sense under conventional economic theory, such as the desire to pursue stable prices, the aversion to Ramsey pricing, and the role of incremental cost as a pricing floor.