Why isn't it fair for the author to be the one who receives the marginal value that they created? They were the ones who created the course, and that course would not have existed for anyone if they put the effort in to build it. Anyone else can do it, but they are the ones who did.

To my eyes it seems very fair to offer a product at (appromimately) the same real price for everyone in the world, and let people choose whether or not to buy it.

And for what it's worth, commodities are also priced at what the market can bear, not at their marginal cost of production. It's just that competition pushes those prices down very close to the marginal cost. This course has competition too, in the form of all the other articles and tutorials that are available free on the internet. It's not like this course is the only gate to otherwise-inaccessible knowledge.