According to Washington state law (
Revised Code of Washington 9.41.250), it is illegal to possess (including own in the privacy of a home) any "spring blade knife," which is defined thus:
The bolded section applies to balisongs and gravity knives, and this meaning has been upheld in court. I should clarify something about "big brother" though. Knife possession regulations as a general rule have very, very low enforcement because they are widely considered antiquated and not terribly important by most cops. Even though this law prohibits ownership, the only time this type of law is enforced (and there is case law to back this up) is if a person carries it in public or in a car under circumstances where the police otherwise became suspicious of their behavior enough to search them, or the less likely scenario of a search warrant executed in one's home, which would indicate one has already committed a pretty serious crime that makes the knife seem petty by comparison. Police cannot detect online purchases of knives nor can they legally search people's mail without a federal warrant, and frankly most cops will agree it is not worth the effort when there are things like drugs, bombs and guns to find.