First of all, I laud Dale's well thought out, intelligent, and respectful contribution to a controversial topic. Thanx!
To attempt to answer your question about who gets to decide what to hack down: the individual, of course. Each situation is different, and ultimately the individual must take the risks that go along with the action. Perhaps the risks are too small in cases where it is outright unethical, I would of course point out. For example, illegal logging by blackmarket outfits looking to make a quick buck. I am not sure what the penalty is for getting caught for such a crime, but it should probably be harsher than it is.
In this situation–Justin and the blackberries–there are some unknowns that make it difficult to have an accurate account of the matter. I am still not sure if it is illegal, and thus, if he would get a fine or whatever. I have a hard time seeing his actions as unethical from what I understand of it (he didn't even cut the blackberry bushes down, but is planning on it). Even if it is illegal, I don't think it ought to be, or that he should have to pay a fine or whatever, unless we are able to prove that he is doing serious damage or is directly harming another person by his actions. The point about erosion is a solid one, as I have already mentioned.
Things like the example you gave are easier to figure out that the snowmobile people should ought not have done that. You can make a bridge out of dead/fallen wood, or go and buy very cheap wood (even get it free from a junk yard or dumpster) to make a bridge. Trees, as I mentioned earlier, are a bigger part of the ecosystem. Blackberries are not as big a part, and from what other people are saying (I am admittedly assuming they are correct that this plant is invasive and a problem plant, again I don't know, don't live there), Justin might even be doing the area a service by removing the plant.
You have your finger on the bigger, larger, more important problem, which is industrial logging. LNT gray areas are the proverbial drop in the bucket next to them, and it is worth mentioning this for a productive discussion (and hopefully more productive actions against bigger threats to the environment). However I have read far, far more posts complaining about LNT (including some very recent ones) on this forum. I have read none about logging, other than Dale's post above. In short, we got bigger fish to fry, people, if we want to preserve the woods so we can continue to enjoy them backpacking/camping.
I also think that some people that like to complain about small stuff like LNT have ulterior motives. There is a sense, a vibe, that some of the people that vent about LNT do it to feel superior than those dumb hicks that chop down trees for bon fires and throw their candy bar wrappers around. While I don't like these actions either, I and every one else in the western world pretty much, is just as guilty if not more so to damaging the environment than those hicks. I don't know much about what the process is to create cuben fiber, but I am guessing it is worse on the earth in general than the trees cut down and the candy bar wrappers left by dumb hicks. Even the vegan, hybrid driving, commune living, tree hugging hippie is guilty to one extent or another–some are even more destructive than the hicks. I see some of these hippies in coffee shops with iPhones, laptops, etc. Very, very few people are off the grid and out of society 100%, and those are the only ones that are not guilty of contributing to all this mess.
We are all guilty, but we ought to do what we can when we can to be less guilty of course–and we ought to also prioritize how we can be less guilty.
Is stopping the chopping blackberries a priority to the environment? If so, why?