False impression, the Confederate flag has always been a flash-point issue in this country. But it doesn't get more local than our own national Civil War does it? So yes, clearly the distance, and moreover the fact of being in a completely different country, helps explain why issues related to our Civil War are super-important to us but completely confounding and esoteric to you.
Manfred made a statement that illustrates this point... he said: "he (the shooter) had a Golds Gym shirt on in the picture (of him waving a Confederate flag) as well. So why not ban that?" Now putting aside the fact that we don't fly "Golds Gym" flags in front of government buildings... his legitimate point was... why focus on one symbol displayed by the shooter and not the other? Perfectly understandable coming from someone who did not grow up learning about US history, Civil War, reconstruction, segregation etc.
The Confederate flag has historically been used in the US as a symbol of the power by white-supremacist movements. It was put up on government buildings in the southern states as a protest against racial integration and Civil Rights for blacks. So the continued presence of it was an indication that white-supremacists continued to have enough power that their symbols of power could not be removed from prominent official display. The fact that the flag has been taken down does not mean that there are suddenly less white-supremacists or that they are less racist all of a sudden, indeed not. But what it does mean, is that they are no longer powerful enough to keep their symbols prominently displayed by the government (at least in SC, we will see about Mississippi). Taking the flag down is a demonstration that white-supremacists are less powerful than they once were.
The connection to the shooter, is that he was a white supremacist. That is also probably why he was taking the kinds of photos he took displaying the flag in the way he did. Leaving the flag flying after that would have indicated that people like the shooter were still powerful enough politically that their beloved symbol of power could not be removed. The fact that it came down was a statement to them, and everyone that they don't own/run SC anymore.