(edit: added note. This post will veer into J.Swift territory at the end. All, understand that that part is a critique of the general approach to terrorism that people tend to espouse these days, not a serious suggestion).
Maybe they should have.
Are you being serious or just yanking a chain or too? No country has anything like a completely monitored border. (OK, I don't know about Monaco, etc, but.....). It isn't financially plausible. If it became so, the marginal cost wouldn't match the marginal risk fought.
For example, the US has tens of thousands of miles of border including every twist and turn. Come to think of it, with twists and turns, it's probably 100,000+ (despite it being a few thousand across). One might cut it out by building one's walls a mile inland, but then one leaves open tunneling. Whatever one does, one is looking at a tremendous amount of territory that would need massive human surveillance, massive technological surveillance, and which would actually drive up the price of goods*. It'd be trillions and trillions and trillions to close our border, and Germany's would be less but still in the trillions.
Compare that to the monetary cost of terrorism.
Now for the non-monetary. Consider how many Americans were killed by terrrorism in the last 20 years, in the US. What is it? 4,000? A few hundred over 9/11? Compare that to the number of people killed by drunk drivers (about 10,000 a year, or 200,000 in that time). Cancer (590,000/year), (600,000 /year)
Over those 20 years:
Hearth Disease: 20,000,000 killed
Cancer: ~ 11,800,000 killed
Drunk drivers: 200,000 killed
Terrorism: 4,000 killed
What's my point?
My point is that we react in massive disproportion to the objective threat posed by terrorism as we do to things that kill vastly greater numbers of people. We propose massive intrusions on privacy and freedom to fight terrorism, but even less - rather than 10-fold or more - intrusions to combat the greatest killers. Freaking out over a border crossing that might even be fake is the least of the symptoms.
Our policies are moronic. If we're going to monitor private emails and texts, if we're going to wiretap people, if we're going to actually ban muslims from entering the states (whether or not from "terrorist" countries to be determined, the label itself in question...) then we should probably just straight-up ban added sugar and execute fat people.
If one threw out emotions and addressed things with cold logic, and if one worked from the starting position that our approach to terrorism is correct, then this is probably too sedate a suggestion. Persons possessing tobacco should be flayed alive, with 1/30th of the skin removed on the hour every hour. Drunk drivers should be drowned in alcohol, but slowly: a 'witch test' in vodka. And so on. Because remember, terrorism killed 4,000 people in 20 years here, and we're talking about two things that killed 20,000,000 and one that did 200,000.
Of course not. Let's incorporate some perspective into our emotionally-driven approach to terrorism.
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*I say that because the reason we don't have stronger immigration policies is the corporations that benefit from illegal labor downstream, where plausible deniability lies.