They've worked for it, so they have every right to do what they want. If a teacher writes a book and sells it via a big name publisher, it's legit, but if they do it online, it's not?
Some of the teachers who taught me had written material that were way better than the books that were recommended for us. If they make a living out of it, why not? Teaching is a low paying job. Might as well make money on the side.
But, as the article says, if people are buying preschool lessons, then it's just lazy parents. Do you really need external lessons to teach your kid to be good at preschool?