Political Situation: The United States begins the game at war with no one. In addition to the normal restrictions (see “Powers Not at War with One Another,” page 15), while it’s not at war with Japan, the United States may not move any units into or through China or end the movement of its sea units in sea zones that are adjacent to Japan-controlled territories. While not at war with Germany or Italy, the United States may end the movement of its sea units on the Europe map only in sea zones that are adjacent to U.S. territories,with one exception: U.S. warships (not transports) may also conduct long-range patrols into sea zone 102
Countries have a limitations on where they can move to while neutral. US has those and a couple additional restrictions on where it’s naval units can go. The whole rule was solely limiting ships from moving around wherever in the European theater. My interpretation, take it for what it’s worth, is that there was no need to limit anything else.
There’s no need to talk about land units since they legally can’t end their turn on foreign soil and the transports that would move them elsewhere are limited in where they can go. Same thing with air units, they can’t land on neutral terr and they have to end their turn on a legal landing space (land or carrier). Limit where the carriers can go and you limit where any air units can go.