I'm young and dumb, so maybe I'm missing something, but it sure seems depressing that standards like HDMI aren't open and free to use.
The problem is that the word standard is used for describing many different things which usually leads to concussions. During the recent years in the open source world, it has become a common thing to call open specifications “standards” which has led to a belief that in general standards are freely available and free to use. On the other hand, hdmi is created and maintained by an association of companies and therefore, they can charge royalties on they users as they own that technology and it is their product indeed (just as ARM sells their IP to manufacturers that want to make processors).
Additionally, ISO standards (or their equivalent ones from the corresponding institution in each country) are more or less specifications recognized by countries (they contain other information such as definitions and tests required to be able to say that your devices complies it) and can be used for some legal purposes such as avoiding some required measurements by using the pretested design they provide. As far as I know, you can implement them without paying anything to the standardization institution, yet they might require the use of a patented technology and therefore paying the corresponding royalties to its holder. Nevertheless, the standardization institution usually sells really expensive copies of them, but they can be found at some libraries.
Standards are indeed a particularly complex thing that changes a lot between countries (for example in my country we have different words for each of those things and they are completely different things according to our law), so the above explanation might not be totally correct, but I hope it helps understand that the reason why hdmi requires paying royalties is because it is a product sold by a “company” instead of something like the C99 ISO standard or python specification