Blending into an innocent crowd doesn't work so well when the government doesn't care if they have the right warm body in a cell, just that they have A warm body in the cell.
And when they're willing to fill a hundred cells with innocent warm bodies in hopes one of them will be the right warm body, such tactics will backfire. I'd say hilariously backfire, but it's not funny at all to the other 99 people.
It is a fact of federal law (statutory) that if a US citizen violates US law while outside of US jurisdiction, they can be prosecuted for that violation when they return to US jurisdiction -- even if what they did was not illegal where they did it.
The US Constitution does not have territorial limits built into it, those have been created by the courts. It is a violation of federal law (Title 18, Chapter 13, Sections 241-242) to violate the rights protected by the Constitution (as well as statutes) under color of law. Doing so alone and unarmed is punishable by a year in federal prison. Doing so with two or more people or with the threat of a dangerous weapon involved increases the punishment to ten years in prison -- a felony by the standard both state and federal governments use to determine such things.
Federal courts and many state courts have ruled that mere possession of a dangerous weapon while committing a crime -- even if the victim never knows the criminal had the weapon -- adds the with-a-weapon enhancement automatically.
Tl;dr: Every time an agent of the US government acts like the Constitution doesn't exist at the border, they became a felon -- but the government didn't see any need to prosecute itself, and ignored the crime.
No, it's not structuring. Structuring occurs when people make bank transactions deliberately below the reporting threshold to avoid the bank making reports.
Asking the bank how much cash they have on hand, and how big a withdrawal you can make without the bank running out of cash and having to call for an armored car delivery, then making that size withdrawal is not illegal.