This is probably one of the more irritating myths about knife laws. Ballistic knifes are not banned in the US. Federal law (15 U.S. Code § 1245) prohibits importing them into the US, selling them over state lines, and manufacturing or possession with intent to sell over state lines. Mere possession or manufacture alone is not illegal under US federal law, but rather is left to the states. And yes many states have restrictions and some outright bans, but there are many states that don't. For example in my own state of MD it is legal to possession them and even carry them in public unconcealed, but they cannot be sold or bartered.
A person in the US wanting to obtain one does have a difficult challenge to overcome, to be fair. Assuming they are legal in their state, they would have to find a knife maker willing to manufacture one for them, or make one themselves.