In nearly every home in the US, let alone the world, the doorways are locked with $5 pieces of tin and maybe a tiny bolt of metal shoved through some wood. There is little challenge to defeat these locks, either through picking or just jostling the door open or breaking the jamb. Furthermore, it's often the case that the doors are not locked at all, or perhaps a window is left open, or unlocked, and it's just assumed that since it's a second story window, that nobody would try it.
So many of these homes are invade by thieves. And yet, there is no question that those invading were violating a law.
If you enter a public place, rules tend to change. Despite the doors not being locked, I can walk into a grocery store and not feel like I've trespassed because it's a business and that's expected. However, I've often seen unmarked doors in dark corners of large stores, or even doors marked "Employee Only" or maybe an unlabeled staircase leading to who-knows-where. I know I'm not welcome in those areas, and if I entered one and was subsequently accosted for it, should I be shocked?
Now we start talking about computers, and their presence on public networks. To me this is some kind of bizarre combination of the two previous physical scenarios. The computers themselves are viewed as having the privacy rights of the house, where-as their offering and the environment in which they make the offer is more like the store, or even another unmentioned public situation: A public park. So how do we come to the conclusions we make? Why is "security by obscurity" not enough to justify criminal charges to those who would violate it?
Or, if you see things the other way, then I ask why you think that the public accessing a publicly offered machine is somehow unlawful, even if they are walking through those otherwise unmarked doors or looking for out-of-the way staircases?
Just because a person doesn't break a lock to get into a home doesn't mean it's not breaking and entering, and just because a door at a store is unmarked doesn't mean the person's trying to break the law either. In the internet, your computer is knowingly placed in the public arena with open attempts at making it easy for the public to find and access, yet somehow accessing an unadvertised part of that computer is a violation?
I don't think the answers are clear but I do think some of the associated assumptions on both sides are questionable. It's interesting to thing about at least. Who has the responsibility here, is it the site admin's responsibility to batten down every hatch or is it reasonable to expect people not to snoop around? You tell me...