My kindle is in airplane mode since I opened its box and I send books to it via usb. No one is forcing you to use amazon services, I didn't even pay for the ad free version but I've never seen an ad.
[0]: https://www.amazon.com/Designing-Data-Intensive-Applications...
Psst, "Designing Data Intensive Applications" was very good read. Do you know similar books that focus on distributed systems?
In principle, you're absolutely right. In practice, .mobi is easy to generate, modify, fold, spindle, and mutilate with free/Free software.
Even Amazon's .azw is just mobi with * replacing $.
While MOBI began as one's company's proprietary solution, the format is well over a decade old now and quite well understood by the Free Software community. Calibre can convert EPUB (or anything else, really) to MOBI, so you can buy or pirate your ebooks from anywhere and easily put them on a Kindle.