[gms7777]

Non-free educational materials aren't charging money for the public knowledge, they're charging money for the effort it took to put the public knowledge in a form that is easy to learn from. This effort is very often non-trivial. The public knowledge is still out there, and the person putting up material behind a paywall isn't subtracting from the availability of that knowledge.

Now, it would certainly be different if the person was literally just copying the documentation, or somebody else's free or non-free materials, because thats just plagiarism. But I think if a person goes to a significant effort to make educational material and wants to charge for it, and there are people to whom this material is valuable to pay for it, then power to them.