It’s not like fencing off part of the sidewalk. It’s like having a building next to that sidewalk with a door. The fact that the door is easily accessible doesn’t mean everyone is welcome to come in. If the door is open that’s generally how it is. If it’s locked, even badly, entering would be trespassing. If it’s locked with a button that unlocks it, and the button says “by pushing this button you agree to the following terms,” well, that’s hard to say.