Would you care to cite this law, please? To the best of my knowledge, rocket engines above a certain thrust level are regulated, but the technology itself not secret.
Just as you said, rockets past a certain threshold are restricted. See the wikipedia page on Model Rocket Motor Classification. Anything past O requires government oversight.
"High-Power rockets in the United States, are only federally regulated in their flight guidelines by the FAA"
the FAA, because they regulate pretty much anything big that flies, they're not a fan of you flying stuff into planes, it makes for depressing headlines.
All you need is a license, a flight plan and proof your contraption is safe, same as a helicopter, plane, or zeppelin.
The blueprints for the custom development of the Saturn V rocket engine are not available to the public, locked away as a state secret.
The plans for NASA's specific rockets are kept confidential yes, but then are the plans for pretty much all commercial craft, that doesn't mean other people can't build planes, helicopters or rockets. You just can't build the governments specific rockets (unless you can somehow reverse engineer one).