Creating an index of these files seems more like driving down the street taking pictures of all the houses with their front doors left open. Something Google seems to get away with on a regular basis.

Trespassing is not a good example. Let's say your private home is (for some reason) in the middle of some downtown area, where every neighbor is a public shop (IE, the expectation is that everything is welcome to the public). Not only that, it's pretty much indistinguishable from the outside from all the other public shops around it. In this scenario, if you leave your door wide open in the middle of the day then no one would take your cries of trespassing seriously.

So no, I don't think trespassing is necessarily still trespassing, even if you leave the front door open. Like most things in life, it depends on context.

The expectation and the norm of this situation is that data published on public ports is for public consumption. You can't blame someone for accessing data that you put out there in a public manner, even unintentionally.