I'm hoping this will be clearer than it currently is in the episode itself, but isn't Rosa Parks and her bus ride an odd event for a villainous time traveller with a nefarious plan to change history to choose? Not trying to take away from the event or its importance, but even if Rosa Parks didn't ride the bus that day, isn't something like this inevitable to happen anyway? It would only have been a matter of time before another black person rode another bus, refused to give up their front seat and get everything moving again anyway. Hell, as it is, there was a similar situation nine years before Rosa Parks had her fateful bus ride, there was
Viola Desmond in Nova Scotia, a situation not dissimilar to Rosa Parks.
Yes, Rosa Parks is an important historical figure and she and what she did should be taught to everyone, and as a historical subject for Doctor Who to cover and teach its audience about, I have no objection to getting an episode about her. But the story logic I'm not quite getting at the moment, though hopefully the episode itself will present things a bit more clearer than the promotional material is, admittedly out of necessity of protecting spoilers.