Even if the door is open you still have no right to enter the house. Same goes with any idiot who leaves their keys in their car, you have no right to steal the car. If it's an ethical question then simply put, it's unethical to trespass on someone else's property whether you share a common link or not. That would be like you having the right to walk into your neighbor's house any time the front door is open, just because you share the same street.....