Getting permission to "explore" is fine, and I'm sure those that you mentioned did just that. However jumping a fence whether you mean harm or not is still trespassing.
The way I was raised was you respected other people and their property. Never mind that they actually have a law against it.
Hard one to explain over this medium as statements get taken as black or white, when they are intended to be grey. I'm sure if we talked it over most people would probably agree on most of the points, but in print you can't pick up the inflections and emphasis. I'll use dead simple bullet points to try and be clear
I'm not condoning trespass, but I am saying I enjoy viewing the results of that trespass, especially because you get to see what is not normally displayed (with someone else taking the legal risk). Vandalism is a different scenario entirely, which in no circumstances should be allowed.
In terms of "respecting property" then the genuine explorers, it can be argued are more respectful than the owners that are allowing it to decay, even though the owners are not comitting an offence but the explorers are. The recent infiltration of river country caused more respect to the site and it's memories than the company do in letting it sit rotting, even though there was probably an offence comitted get the pictures. People are clearly interested in the half finished pop buildings, they have a right to be interested, but not to trespass to satisfy that interest. The company also has a right to say keep out, and wait till we're ready to complete them.