How is the U.S government like a monopoly?

According to the US constitution, certain functions and responsibilities are a monopoly of the US government. These include a national military, the right to wage war, tarriffs and duties, national elections, national taxes, issuing coinage and currency, etc. What's left is allowed to the states, but often there are disagreements as to where states rights prevail (This was the main issue of the civil war). It would not be a mistake to state that all national governments reserve certain monopoly powers for themselves in one way or another.

There are certain things that only the federal government is allowed to do, such as print money, or hold nuclear weapons.

It's more like a franchise than a true monopoly, though, as officials are elected, and therefore are not completely unaffected by market forces.

Its not