Here’s what our actual laws against booby traps entail: "A booby trap may be defined as any concealed or camouflaged device designed to cause bodily injury when triggered by any action of a person making contact with the device. This term includes guns, ammunition, or explosive devices attached to trip wires or other triggering mechanisms, sharpened stakes, nails, spikes, electrical devices, lines or wires with hooks attached, and devices for the production of toxic fumes or gases." https://definitions.uslegal.com/b/booby-traps/

So yeah, actual booby traps are illegal, but a concrete mailbox is not necessarily a booby trap. A lawsuit arguing a concrete mailbox is a booby trap is an attempt to classify it as booby trap, which means it is not currently classified as one.