Interesting. Show us your train of thought about how the Constitution supports confiscating cash from private citizens. It must be some indirect way, as through laws passed by congress (I'm guessing).
Article I, Section 8:
"The Congress shall have Power To lay and collect Taxes, Duties, Imposts and Excises, to pay the Debts and provide for the common Defence and general Welfare of the United States
...
To regulate Commerce with foreign Nations"
Now, I know I've offended some of the open border types on this thread, but I don't really care. This is a core precept of the Constitution.
These were international travelers, not citizens.