"Providing for security is also inherent to sovereignty of nations, but private security firms flourish"

Security for a nation state is different to a security guard standing in a shop. Also I'm quite sure that nation states reserve the right to claim a monopoly on certain parts of the security industry. The monetary/security power is in being able to claim that monopoly, as your currency example shows, the nation state can allow certain behaviours, that doesn't mean it has to allow everything.