Something being accessible does not mean you're authorized to access it. Someones house being unlocked doesn't mean it's okay for you to enter. Authorization is the key part here and you likely can be convicted under the CFAA[1].
Again, I don't think the analogy holds—no one is entering anything. A better analogy would be someone standing outside your house and asking you to pass a book to them through the open door, which you then voluntarily do.