You have no idea what you're talking about. Paid online courses are also required to make their courses accessible to students, what's different for them is they can make a course accessible to each specific student which can be easier (e.g. if a student is deaf, they need captions but not written descriptions of visuals). Most cases involved brick 'n mortar businesses and institutions and in many online cases they've been extensions of the physical world (Target, UC Berkeley). More recent cases (Netflix) have made it clear that online-only entities are also considered places of public accommodation.
Why UC Berkeley this time? I don't know, other than the complainants came to the DoJ about them, and not some other school.
The purpose of the letter is not to cost the University or to stop them making the materials available. The DoJ never suggests that, it's Berkeley that raised the idea on their own. The purpose is to explain how the University has been violating the law and what they can to correct it so ultimately what is offered can also be used by people with disabilities.