Natural Monoplies being 'harmless' is not standard economic theory.

http://en.wikipedia.org/wiki/Natural_monopoly

That wikipedia page goes to some lengths to accommodate the libertarian views (which I'm guessing you lean towards) while covering the standard view.

A quick review of Industrial Organisation on Google Books seems to show it reflecting the standard themes on natural monopolies, that nationalization or regulation are the standard remedies and that neither are without problems.

The more-mainstream free market argument is that no regulation of monopolies creates problems, but those problems are less than the problems created by nationalization or regulation. Pretending there is no problem with monopolies is a bit extreme for my tastes, though it seems to get mentioned a lot in the US at least.