The question remains what their actual task is. Is it to protect the citizens or companies of Hong Kong form something? Is it to retain control over the supply of financial vehicles - as ordered by the government? If it indeed was one of those things, they effectively are obliged to take a stand on this.
I don't think they have any explicit task to do except manage the supply of Honk Kong Dollars, and the tasks associated with it.
What they did was probably just a self initiated PR move to show Hong Kongers "The central Bank cares about us."