Not to defend Wolfram, but to understand this you'll have to understand that "The Wolfram Language" is not like the traditional "programming languages". Traditional programming languages consist of a set of grammar, essential built-in libraries and a few development tools. The Wolfram Language is more than that. It includes the Mathematica language, a very rich set of libraries/algorithms, rich bodies of knowledge encoded in curated datasets in various kinds of physical and social sciences.

It might be best to compare the Wolfram Language to natural languages like English instead of programming language. The English language is not just a set of grammar, but also a rich collection of vocabularies that need to be compiled and maintained, and perhaps a rich collection of literature that need to be maintained and expanded to help learners understand the language.

Wolfram spent quite some time and resources to collect data for his Wolfram Alpha, and his new "language". It makes sense to charge money for such efforts.