While I understand your sentiment and agree with it to some extent, the other side of the argument is that by releasing a free course, you disadvantage someone who would create this content, for a fee, and use that money to make it accessible to all. In effect, they are giving a free advantage to those without disabilities, making it even harder for those with disabilities to compete. Is that fair?
An analogy appropriate to this site is a free app that does an adequate, but not great job for a given need, that prevents others from creating (or maintaining) a polished app for money. That is, we might get 80% of a great app with the last 20%, the hard part, never being achieved. It is what it is in a free market, but the government has a bit of a mandate to try to take the larger perspective and create pathways to cover that last 20%, at least in the case of accessibility.
Having said that, I feel a better approach would be to provide grants or services to support making such content accessible. There is something odd about having to tell people that not only do they need to be an expert in a subject but also in how to convey that in a way that is useful and good to the people who are blind, deaf, or manually impaired.
By this I mean, for example, why is it clear that a simple transcript on a video is suitable? Surely it would be better to design courses to take advantage of the abilities of those using it and compensating for the lack of certain abilities. It is not clear to me that simply captioning a video is the best outcome for providing materials to deaf people. It would be far better to encourage and fund people expert in teaching deaf people or blind people to create materials directly, perhaps based on these other free materials.
As a creator of an online math course, one in which heavy use is made of graphs, it is not clear to me how I could even begin teaching graphing to blind people. I simply do not have the knowledge or experience to do so.