> a statement as simple as 'black lives matter' was controversial.

That was never controversial. What was controversial was the unstated implication after that statement: "and people don't think they do". When your slogan is a false accusation against people in general, they aren't going to take it well. Approximately zero people in the US think black lives don't matter, so they aren't going to appreciate you coming at them with that accusation even if it is left unstated.