Malbim: "B'Yom Tzeischa v'Avarta Es Nachal Kidron" implies that he was adamant only about going in that direction; the road to Bachurim was there (Shmuel II, 15:23). However, "do not go to and fro" (verse 36) implies that he may not leave in any direction! It depends on Shlomo's reason. If it was like incarceration, the Isur is in any direction. If it is lest he rebel, the Isur is only to go towards his Shevet, where he could arouse a rebellion.