Terribly unethical, yes, but I can't really think of any legal way to address it. In fact, the high-fencers might actually be right - not just absent wrongdoing, but supported by positive law. This is because traditionally, things in nature require some effort to convert them to one's own property.
http://en.wikipedia.org/wiki/Pierson_v._Post That's the standard first-year property reading on the issue. While I'm sure it's spiraled in many directions I don't care to research right now, that's your basic foundation - the one who deprives the animal of its liberty is the rightful possessor. By erecting the high fence and going to the effort of capturing the deer, the ranchers would seem to satisfy that test. Traditional law of capture is the umbrella under which PvP would fall, and would typically support this finding.
http://en.wikipedia.org/wiki/Law_of_capture
That is not to say there aren't equitable complications on the issue. In a time where deer populations can directly affect one's property value if it was purchased for or might be sold as hunting land, there is clearly some connection between "hoarding" the lands' resources. The law of capture has been obviated by state legislatures in a number of ways (see above wiki article in general). If this becomes a big enough problem, perhaps deer populations could be estimated and prorated where fencing occurs, although even then, you run into the problem of rancher potentially keeping only the quality trophies and releasing sub-par animals to meet per-area quotas.
Further, I doubt it would be easy to get legislatures motivated, especially in states where big-city politics steamroll concerns relating to non-hippie conservation efforts. Therefore, a private solution is probably the best option, at least for the time-being. A neighbor could contract with the rancher, or erect their own mirroring gates. But this does further harm to one who is seeking to reduce their harm (costs incurred) already. So perhaps, provided the ranchers' neighbors have adequate acreage, the best option would be to obstruct the ranchers' use of those gates by interfering with the deer's travel there. You obviously can't block the gates directly if they are off your property, but you could block the usual deer travel paths, install noise or scare devices, put down the hair or waste of predatory animals, frequent the areas yourself and leave scent behind, etc. Maybe even set traps of your own with more attractive bait more available to the deer near the gates, trap the deer, and then scare the piss out of them before you release them so they never come back to that gate again. Not sure how the DNR would feel about that last option though. Sucks, but that's about all I can think of (that wouldn't be terribly expensive) given the current state of the law.
Definitely interesting to see how this one plays out.
