There are many schools of thought with bonsai. Very many people use "clip and grow" techniques. Wiring techniques are more popular in Japan than in China from what I have heard. I'm not that experienced with bonsai, but the people I have talked to are not so keen on the production of artificial "jin" (bleached dead wood). If there are dead parts of your tree, you need to do it to keep the tree healthy. An abundance of jin seems to be more of an American style than Japanese in my limited experience.
In any case, I don't think the treatment that bonsai trees undergo is harmful to them. In fact, they are arguably much healthier and "happier" than a tree that's left to grow. The pruning of the roots makes them incredibly vigorous because they have many more feeding roots for their size than normal. You have to baby these trees too (i.e., water it every day) because the soil can dry out very quickly (there isn't much of it). So inevitably they are free of disease and insect infestations.
If you spend some time around well cared for bonsai it's pretty obvious how healthy they are. I see no evidence at all that the trees suffer. Quite the opposite. I recommend learning more about it if you are interested in the welfare of the trees (something that I won't criticize anyone for).
Also, I had thought these trees were grown from relatively normal saplings, not from already unusual wild trees. I see such trees growing in extreme conditions, usually at the windswept treeline of a mountain while hiking. I would not be happy to see some "artist" digging them up.
Comparisons to other human activity are only going to make it seem comparatively innocuous and there's a gulf of difference between the relative and the absolute.
[1] honestly the alive/not alive bit isn't even strictly that important. People are often fairly opposed to destroying various rock features, even when it's for the purpose of a very practical and utilitarian dam. Somethings resonate with people in a way that they want that something to be allowed to just be
So the difference is purely about aesthetics, which brings us back to why people do bonsai...
Though it's worth pointing that most people care about the difference between killing something and controlling every aspect of that things life.
I read your full comment, and I don't really care what you care about, my point was to point out that your point was inconsistent, whether you were expressing it as your "own" point of view or a "friend of a friends" I don't care, the point is still inconsistent.
As I said previously, bonsai trees are not starved of resources. Quite the opposite. They are specifically pruned in a way so that the tree is able to absorb nutrients much, much better than they would if left unpruned.
You are probably wondering why they trees are small if they aren't being deprived of something. I invite you to spend some time learning about horticulture if you are truly interested in the answer.
ME, personally, yeah I contribute to a lot of ethically horrifying things through my economic choices. Mostly because I'm lazy, also not sufficiently rich that I could afford to consume only things I find ethical even if I was less lazy.
Even if I could control my reactions before I even knew I was going to have them I don't see my lack of action with regards to some feelings of revulsion (I eat eggs despite egg farms being horrifying) as a reason not be uncomfortable about stuff.