Assisted Opening knives were designed to exploit a loophole in the language of most knife laws, including the federal one. 15 U.S.C 1241 (the US-wide federal law) defines a switchblade as:
any knife having a blade which opens automatically(1) by hand pressure applied to a button or other device in the handle of the knife
Emphasis added.
Most state laws copy this language almost word for word in their definitions. What sets the Assisted Opening knife apart is that it has no device or actuator located
in the handle. The operation of the blade's movement is by pushing the blade itself, just like a normal one-handed thumbstud knife. Therefore, it doesn't count as a switchblade under most (but not necessarily all) state and federal definitions.
Why didn't they just rewrite or reinterpret the law to cover AOs too? I think it's because most people around the time of the AOs break into the market realized the old switchblade bans were stupid, knee jerk laws based on watching too many gang movies, and that the fact that handguns are readily available throughout the US made arbitrary restrictions on certain knife mechanism rather foolish and pointless.
Given that, you may then ask "Why didn't they just repeal the switchblade bans?" Well, in some states they
have repealed them (NH, AZ). Just that repealing old laws is very difficult to do, often the legislature has their hands full with other more pressing matters (opinions vary on this).