When you look at the details it isn't just BLM. It's any human rights activism.

Two angles I see: 1.) China has been VERY sensitive about it since the Hong Kong protests and people slipping "free Hong Kong" in everywhere. 2.) The Olympics have always been about nations engaging in their rivalries on an "even" (if you ignore doping lol) playing field versus their rivals in sport. Soviets versus the US comes to mind. Diverting from that to have all the participants grandstand their internal and external politics in the sideshow stuff is antithetical to the purpose.

Now, given what we know about the global community, this is far more likely about the first one, IMO.