It is illegal to import a balisong knife into the United States. US Customs Regulations consider them switchblades and illegal to import.
Customs faces two problems: first, they've got bigger worries than balisong knives. Frankly, they just don't put much emphasis on enforcing this rule. They're more concerned about terrorists with trunks full of explosives, drug dealers with tons of white powder, and with politically sensitive issues like fur or other material from endangered species.
Second, there is no easy technical way to detect balisongs. Gas chromatagraphs, proto emission scanners, and imaging x-ray machines don't easily distinguish balisongs and attempts to train balisong sniffing dogs have so far failed.
So, enforcement is spotty at best. But, a noted French balisong artist had to borrow some of my balisongs in Atlanta after customs confiscated his as he entered the country.
Also, most of the major US knife shows take place in cities where balisongs are illegal. In fact, the only major exception that I can think of is the Oregon Knife Collector's Association show in Eugene, Oregon.
Enforcement is also spotty. Clay and I entertained a delegation of Atlanta's finest police officers last year. Let's face it: the city wants the tourist money and as long as nothing gets to out-of-hand, they're willing to bend the rules a bit for a weekend. But, it can be an uncomfortable position for some exhibitors.
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Chuck
Balisongs -- because it don't mean a thing if it ain't got that swing!
http://www.balisongcollector.com