Ever tried to send money to Somalia? If you think that's a silly question, there are a lot of Somalians here in Minneapolis who would like to feed their families back home (and a lot of banks who would like to make money by helping out). But politics makes that very, very hard. This is just one example.
[0] - https://ripple.com/
I'm not making up issues, you know. Inability to move money to Somalia is a real thing. Here's one link: http://www.startribune.com/local/minneapolis/136396028.html
Whether those regulations are actually effective is questionable (see HSBC, BoA, etc.), but that's the rationale.
Edit: Did a little bit of digging[1], looks like they accept wire transfers (e.g. Western Union), and I'm sure there would be ways through those e-currency and other services to deposit large amounts of unregulated money.
[1]https://support.mtgox.com/entries/20490576-Withdrawals-and-D...
Money in Saudi Arabia -> mt gox -> bitcoin -> mt gox -> terrorists inside the USAOr actually just kill people with pistols. Seung-Hui Cho, the Virginia Tech mass killer, kills 35 people with his Glock 19 and Walther P22.
The attacker is basically undetectable until he start his assault, and the probability of success is quite high. He can even use his car to ram into the mall door and run down individuals before he start his shooting spree. The only hope for civilians at that point is that a policeman or a civilian with a pistol to start shooting back.
The operation can be done with the attacker saving money for those purchase of weapons by working at his usual job. You don't need to send funds.
(Of course, if it was that easy, why don't terrorists do it more often?)
It isn't, which is why there are big stacks of anti-money-laundering regulations that governments impose on banking and forex.
I have personally discussed this with a friend that is a federal reserve spokesman, so anyone that wants to argue otherwise should consider that first.
A major federal reserve branch maybe has $30 billion in cash on hand at any given time. The marginalization of the US cash market is almost complete after the past decade.
Lastly, consider the possiblity that those back scatter xray machines aren't about weapons.