Let’s say some of the people on the bus behind them are unaware of whatever injustice (I truly don’t know what is going on). Are they on the side of the oppressor?
Discussion
The quote is by Desmond Tutu (https://en.m.wikipedia.org/wiki/Desmond_Tutu)
Today, neglecting a situation of injustice in context of African Anti-Apartheit history would be foolish. In that time, there could be people unaware, neutral or even actively supporting the oppressors. The picture seems to be later, so yes there could have been people unaware of injustice caused by colonialism, sitting in the bus behind. They were accepting the status quo, because it was indifferent to them. It is surely not possible to be aware of every injustice in this world. Also it might be legit to say "It is not my problem, but THEIRS".
All a matter of perspective and belief onto humanity. If there is no feeling of connection to other ethnicities (or races how it is called in some places) then of course, there can be no empathy, thus no interest to see from an oppressed perspective.