In my experience, the "contamination" factor is mostly semantics. If they consider wood a contaminant, it is. That doesn't mean that there couldn't concieveably be something of a chemical nature IN the tree that would be like what most of us think of as contamination.
As far as the Nitrogen robbing, it's probably not a real issue. First, the concept is not proven, despite some heavy pronouncements by folks like Univ. Maryland Dr. Goins. Second, the cycle is mostly a closed one in the immediate environment of the dump site. If N is locked up, it's released pretty quickly as the composting slows and stops. The concern is usually for the plants sharing the site, and this is not a garden we're talking about, but a place where somebody happily drives a truck up, dumps x yards of chips, and then knocks the pile over with a tractor.
What the property owners are getting is a wonderful site to put an organic garden of monstrous scale. (but don't tellthem that or you might lose your dumping rights anyway!)
Luck!