I can confirm what BenC-60 said is accurate. It all depends on your state. Some states have zero restrictions whatsoever on automatic knives for adults, some prohibit all possession even for law enforcement and military unless directly engaged in military duty.
And yes one component is the federal law as mentioned, which only restricts sale over state lines. I did want to clarify this law though. By the letter of the law, this only permits sale to the military within the terms of an official contract. It does not permit sale over state lines to law enforcement or direct sale to military members as individual buyers. Any company that does otherwise is breaking the law. There are many sellers that, due to having no actual legal department, just kinda make the rules up on who they will sell to. Some sell to anyone. Is it breaking law? Yes. But the feds usually have better things to do so it's almost never enforced.
Did also want to clarify, an assisted opening is not a switchblade and is not subject to the same restrictions, both for federal and state. A knife being OTF or side-opening has nothing to do with whether it's a switchblade. Some OTFs are assisted opening and some are manually operated knives. OTFs are just more common as switchblades.