> You'll find that, often, if not always, if you buy directly from a publisher, the ebook will be free with the paperback purchase. (Manning is one example, not sure about others off hand.)
Extrapolating “often, if not always” from one niche publisher is quite a stretch.
Random House, for example, does not do this.
https://www.penguinrandomhouse.com/books/713318/wonka-by-roa...
Neither does Simon & Schuster.
https://www.simonandschuster.com/books/Icebreaker/Hannah-Gra...
You're right. I answered in the context of technical books, where you do tend to get a free ebook if you buy the paperback directly from the publisher. Since that response, I checked a few of the other publishers and it's correct.
But, re-reading the top question now, I see that it's not specifically about technical literature.