My point was that, in the case of something which requires a continuous time to complete, it would indeed be the responsibility of the creator of the media if engaging with it caused problems like eye strain. Examples would be cases like videos that could not be paused and had to be watched all at once, long levels that cannot be saved midway through in a video game, or something like that. In that case, if someone got eye strain or another concern from completing it in one go because completing it in one go was the only option provided, it is sensible to lay the blame at the hands of whoever set it up that way.
Books do not work like this. Books can be paused and put down at any time, in the middle of a chapter, in the middle of a sentence, whenever. If someone complained to the author that "The chapter was so long that I got eye strain reading it all at once", the author would be completely within rights to say "Then why did you read it all at once? That was your choice, not mine." The cutscene analogy was a case in which it was not the other's choice, and is thus fundamentally different.
I maintain: It's an unneeded restriction, and the justification of "it prevents eye strain for readers" is silly because the reader has the power in their own hands to read in as long or as short sessions as they wish. Putting the impetus on the author to write shorter chapters wouldn't help anyway, since, as you said, you would just split the chapter into smaller pieces and then it would still be the reader's decision whether to read them all at once (causing eye strain) or breaks in between (not causing eye strain).
