I've been able to find only one law that requires people entering the US to use designated border crossing points, which is 8 USC 1325. This law applies only to aliens, however; both relevant subsections of the code begin with "Any alien who...."
Is there a US law that requires US citizens to use designated ports of entry when entering the US?
Is there a law requiring US citizens to present themselves for immigration inspection (or customs inspection) if they cross the border somewhere other than a designated port of entry?