They do at least for Ubuntu. One local to each AWS region even, not just one. Bandwidth is expensive, it's all in their interest to have as much locally as possible than go out for mirrors. That definitely looks like something broke.
Those could very well be a bad batch of AMIs and now that they've all been spun up as instances there's no taking it back short of emailing customers and politely asking them to fix the mirrors.
Or people are just following online guides and adding that particular repo copy pasting the mirror line which goes to the public mirrors.