If you buy my book directly from my publisher's website, I'm extremely grateful. I get a fair amount for that. If you buy it from a local bookstore, at least they benefit.
But if you buy it from Amazon, you might as well just get it from Anna's Archive. At least you're not supporting the jungle.
This is why experiments like ONDC[1] (being pushed by the Indian Govt) are interesting. They want an alternative path between producer and consumer with out a middleman (Platform). Similar to how email works based on open protocols. Or UPI which has reduced dependence on Mastercard/Visa.
[0] https://en.wikipedia.org/wiki/Platform_economy [1] https://ondc.org/learn-about-ondc/
I have no idea how publishers feel about used book stores.
But personally, when I went to a local library, saw that they had two copies of my first book, and both were checked out, I was incredibly proud. I love libraries. I guess overall, I would make more money if everybody who checked out my book at the library bought it instead, but I don't care.
Tech books tend to not sell very high numbers (a book on a niche programming language is considered quite successful if it sells 10k copies), so it's important to make a good amount of money on each individual sale.
So for me personally, Kindle sales in particular are essentially worthless. Let's say I end up making 2 bucks on a Kindle sale (which is more than I make on average). If I sold 10k Kindle books, that would be 20k income for me, which is simply nowhere near enough to justify the effort of writing a book.
The 70% royalty is only available to books below $10, and also incurs other costs from Amazon. Which makes the calculation even worse, because now you're forced to sell at a lower cost, and you pay additional flat fees to Amazon for each sale.
In your case, if you sell the book for $10, you get about $6 ($7 minus fees), of which you get 25%, so you also only end up with about $1.50 per sale.
What my publisher does is sell books on Amazon, and then put a page in it to tell people to please buy the next book directly on the publisher's site. They believe that this is overall the least bad option.
I think it wouldn't be too difficult to make a system for authors to sell their e-books and physical books themselves on their own websites, but the problem is that authors would rage with fury for paying an upfront cost or paying 10% for such a system, and instead stay with the publishers to rip them off instead. You see this in every industry.