A wide variety of laws regulates banks, money transmitters, currencies, and securities—but none of these categories are a good fit for bitcoins.
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http://arstechnica.com/tech-policy/2012/08/why-bitcoin-lives-in-a-legal-gray-area/Because at U.S. border crossings and at customs you must disclose if you are carrying $10K or more in cash outside the country, prepaid cards were used to transfer greater amounts and because they weren't "cash" they didn't need to be declared.
That was described as a hole and then it took over ten years before any regulations imposed any restrictions. (The difference now is that prepaid cards sold are for domestic use only, and a separate product line of cards are sold for use internationally requires AML/KYC.)
