Final answer:
The U.S. Post Office has a monopoly due to legal barriers to entry, which prevent other organizations from delivering first-class mail. This is a socially beneficial measure as it ensures that customers have access to an appropriate amount of mail services. Legal monopolies are often subject to economies of scale, making it more efficient to have only one provider.
Step-by-step explanation:
A legal monopoly is a situation where the government prohibits or limits competition for certain products or services. In the case of the U.S. Post Office, it has a legal monopoly on delivering first-class mail. This is due to legal barriers to entry set by the government, which prevent other organizations from delivering first-class mail. Additionally, legal monopolies are often subject to economies of scale, which means that it is more efficient and cost-effective to have only one provider.