Also, while we may see $75 or $150 or whatever as throwaway money (especially in the context of professional help products or books), that price is wildly unaffordable for most of the planet. Having high-quality free educational material out there is a godsend for students and developers in low income countries.
The Art of PostgreSQL is great and super thorough. Knowing the author I can't fault him on price for the effort he put into it.
But also your point of students and developers in low income countries completely understand that side. It's a shameless plug but we built our Postgres playground with tutorials on a number of topics which are completely free aiming to help target some of that audience - https://www.crunchydata.com/developers/tutorials
MySQL mingles its custom language with SQL which is a common point of confusion... Whom amongst us wasn't confused when "DESCRIBE" didn't work in SQLite or Postgres?
Are they really different technologies? It has always been my impression that they are the same tech, but they have a slightly different syntax in some places (DESCRIBE) and different levels of capabilities (Postgres offers more).
If you know the core concepts of one, you are probably 80% good to go with the other.