Earlier quoted context omitted.
As a real-world analogue: you can indeed be guilty of trespassing on someone's property even if you don't have to jump over any fences or pick any locks to get there. In some places, they don't even have to have a "no trespassing" sign. Simply being present on someone else's property without an invitation from them is illegal, and no, an open door does not count as an invitation.
You generally can’t be charged with trespass unless you refuse to leave when told to do so. An open door to a home is different, but unfenced property is 100% not trespass until you refuse to leave.
It's even possible to be guilty of trespass even if you weren't aware that you weren't allowed on the land. This is negligent trespassing.