I think this is a phenomenally interesting point of view.

It is true that 'boorus' do not bother with artist consent, and I think by and large the sentiment from Japanese artists has been fairly negative, if a bit muted (perhaps this is a bit of a cultural thing?) Despite that, the dynamic between boorus and content creators has always been complicated.

Boorus have some positives even for artists. For one thing, they organize images pretty extensively; it is not abnormal to see a booru post with 50 to 100 tags, and it's probably fairly uncommon to see less than 10 tags - so they're great for searching for images. Because of that, they are an excellent place to search for inspiration or references, and I definitely know folks that do this. They also act as content aggregates, which does help people discover content and artists, especially combined with tags. Boorus do tend to comply with "do not post" requests, though that obviously does not mean artists are then implicitly consenting to their work being posted or used.

But of course, the boorus themselves are not charities. Boorus tend to run ads or even accept direct payment from users in exchange for features. Personally, I do find this unethical, and I highly recommend you do not browse a booru without an adblocker, not even because of ethical concerns but due to the fact that many of them have pretty nasty advertising (Also, be aware that many boorus, like Gelbooru for example, are not particularly shameful about explicit content, and neither are their ads.)

I was involved in a small scale booru years ago in my youth, for a particular interest. It's funny, I was so wrapped up in the utility of organizing and categorizing images that I had not even considered the issue of artist consent or copyright (I was not making any money, in my case, at the very least.) Ultimately, an artist complained that their work was repeatedly posted without proper links, something we did our best to discourage, and I shut the whole thing down in short order, for better or worse.

Like many things on the internet, boorus would not be nearly as useful without their flagrant disregard for copyright. It's not just booru owners in this debate, though - some online artists take a view that copying and sharing online is both inevitable and the point, while others, probably moreso these days due to an increase in bad actors, take a hardline stance against copyright infringement. The law clearly and plainly sides with the latter, and I think I do too. Before sites like Pixiv existed, there were very few resources for finding Japanese illustrations in one place, much of it scattered around in fc2 blogs and Geocities sites (Yes, even until pretty recently! Geocities Japan outlived many other Geocities regions.) Nowadays, there's not nearly as bad of a discovery problem with Japanese art, and the boorus feel more parasitic than they once did.

(Aside: I think many software engineers are used to being very technical about copyright, even when they do not understand the details correctly. Most artists I know do not dabble much into the details of copyright or licensing, and may even have some difficulties grasping the implications of say, Creative Commons terms. It's important to recognize that not everyone has the same point of view and things that seem obvious to us may not even make sense to others.)

Of course, though, that a booru is basically massive copyright infringement is one thing. Now we're talking about neural networks built from them. And damn, that is complicated, and probably breaking new ground. I'm sure people with strong opinions will claim that it is very black and white, but I can't see this as anything less than a dilemma. I agree the model clearly constitutes copyright infringement if redistributed, but the results... that seems deeply complicated.

On one hand, for all of their impressive strides as of late, current neural network algorithms are not really that impressive in their creativity. It's clear they are very heavily influenced by the data sets. That said, though... at the end of the day, our brains are also 'neural nets.' If we design a sufficiently advanced neural net and feed it a handful of pictures from Danbooru, and it is able to spit out similar but clearly distinct art, how is that any more copyright infringement than a human artist that draws inspiration from the same images?

I think people predicted that the copyright situation would get complicated when DeepDream showed up, but it's nothing compared to the calamity that could occur as a result of this kind of data set.