Why do governments regulate natural monopolies?

Certain types of monopolies exist and are allowed in a free market system. Here are some examples:

* When a patent is granted to, as example, a drug company, for a new drug, the company has sole rights to the manufacture of the drug for 17 years. Thus this company has a legal and natural monopoly;

* A key resource is owned by a single company. A clear instance of this is found by examining the De Beers company. De beers owns over 80% of the world's diamond mines and production. This is a rare form of monopoly however; and

* A natural monopoly begins when a single company can supply an entire market at a lower cost than can two or more other companies. Common examples of this are utility companies. It should be noted however, that in these cases, a regulatory body must give the utility permission to raise prices.