> Why is complicity in abuse of populations suddenly a problem?
It's always been a problem, but it's best to make whatever strides to resolve it that we can.
> The US imprisons more than China does
Sure, but it is not currently engaged in a genocide. Standards for due process are also stronger in the US than in China, and you don't get thrown in jail for criticizing the government either.
The US prison system being bad does not make China's abuses any less serious. They are still much worse than what the US does.