Except this isn't indexing, this is creating a replacement frontend.
> There's also the argument that if something isn't forbidden, it should be allowed
That's a really fun legal argument but a pretty irrelevant ethical one.
> replacement frontend
No, they are not "replacement frontends". Those are known as "3rd party clients". These are not clients but clones. They are cloning all the content. That means they've replicated the backend, the frontend, and the database full of content, by scraping/pirating it from the original source.
old.reddit.com is an "alternative frontend". Clones of scraped content are not.
I don't know why people decided to jump on this one specific project but there have been numerous replacement frontends for HN published over the years, and put on Show HN. And clones of HN. And datasets of HN comments.