According to SurvivalLife.com.the states that have specific laws making butterfly knives illegal are Hawaii, Maine, New Mexico and Wisconsin. Many other states have laws restricting these knives to varying degrees.
A butterfly knife, also called a Balisong knife, is considered dangerous and illegal in some states because it can be easily used as a deadly weapon. In some states, including Tennessee and Washington, the laws regarding these knives are a gray area, as the legality of them depends on whether or not they are classified as switchblades. In Wyoming, North Dakota and Oklahoma, these knives are legal as long as they are not concealed. Some states limit who can own them, such as in New Hampshire where they are illegal for felons convicted of crimes against people or property and New York where non-citizens cannot carry them. New York also may require owners to have proper hunting or fishing permits in order to carry these knives.
Anyone who owns a butterfly or Balisong knife must be aware of their state's laws and the laws of any state they might be traveling to. Even having one of these knives in the car when crossing state lines could put that person at risk of prosecution.