Comment by themulticaster

IANAL and I am not too familiar with the US constitution, but the paragraph you mentioned seems to have a very clear intention: To forbid the individual states from a) performing diplomacy/foreign policy on their own and b) creating their own currency.

At this point, I want to point out that while a lot of discussion always surrounds the status and the individual rights of the states, I think it really is clear that the US is the state (as in nation) and the individual states, while they have some autonomy, are the inseparable parts that the US is made of, just like in other federal states (nations).

From that perspective, I think the justification for the paragraph you mention is clear: The states are not independent, and therefore they can't have diplomatic ties with other nations. In diplomacy, this has always been the case: Diplomatic ties always exist between independent nations, never between an independent nation and a federal, dependent state. There is nothing like an "Embassy of the U.S. State of Texas to the United Mexican States" since the State of Texas can't have diplomatic ties on its own.

What you mention regarding printing more money does not apply here: The paragraph you mention talks about the US states, but not about the (nation) state itself, the USA. There exists no alliance of states, rather there is a federal republic consisting of states (which in turn are not allowed to have any diplomatic ties of their own). The (nation) state represented by the US government is obviously not a (federal) state inside the US, that would be absurd.