The problem is that Beijing has a strange definition of what is National Security. It include suggestions (even if constructive and well-meaning), and also journalism (when the facts do not paint a pretty picture). Where other jurisdiction only proceed to prosecution when there are material action and consequences, China is content to prosecute just on the feeling of being criticised or threatened or citizens just behaving rebelliously -- within the first day HK is already arresting citizens for having banners in their bag and for shouting slogans.