Haven't the Olympics never allowed any type of political statement to be made? It makes sense, there's 196 countries competing, all with their own social issues and agendas to push. If you allowed even one voice the ability to express politically, you'd no longer be an apolitical event organizer dealing with the complexity of international relations on a global scale. Other countries or political groups would also wish to express their views during the games, and if you don't grant that you'd risk losing one of the fair few global events that unites humanity together.
I support BLM and have attended marches, but for the Olympics I understand why they wont allow it. It's for the same reason that even during war, there's such a thing as Olympic truce where for the duration of the games countries are expected to remain neutral. The Olympics has to be a neutral apolitical event, and unfortunately that means not allowing the expression of issues for positive societal change, not just the negative ones.