You can't go in, because story-wise you're forbidden. Sure, they could have taken the time to have you completely derail the game by going somewhere forbidden by law, and ostracizing the entire community you are about to play with, but that'd be a huge waste of material for literally 20 minutes of play time.
No, there's not really any defending this. A game touted as "open world" shouldn't have invisible walls. If they want you to not reach a certain place, there are plenty of better ways to do it.
Because you haven't finished the tutorial? Are you angry that you can't get off the boat in Morrowind before you talk to the guard who's escorting you, too? How un-open-world. ---
