Killgar's on to something and I have found similar in my own research. CA does lump them in with switchblades (blades over 2" mind you), but it only restricts carry and sale, not buying/receiving or ownership.
The federal law is kind of an interesting thing in that doesn't expressly lump balisongs in with switchblades, just uses language that might apply. Whether it does or it doesn't, it only affects the seller, not you the buyer. Furthermore, I have noticed that in practice that authorities seem to ignore balisongs even where they would go after switchblades, and there seem to be a a far greater number of sellers willing to risk sending balisongs vs. sending switchblades.
When all's said and done, you the buyer have nothing you should be concerned about other than getting swindled by a merchant that takes your money.