You should be asking, why are Japan and western countries open to these foreign interest group and their internal politics are still relatively stable. On the other hand, why do you think your country is forced to cut down and close the doors for these foreign group because of the fear of internal instability?
I know this question is not directed at me. But still it is an interesting question, which refers to the core of the all issues: The question of world view or ideology.
You see, the West is more or less one and the same (a unified bloc) and Japan is firmly attached to that system of values. Good or bad, it is their decision. The West cannot be a threat to Japan so long as Japan remains a loyal attachment.
But you know, the West does not represent the "international" or the "universal." They just represent a certain portion of humanity and world's geography. The values they boast about may or may not be embraced by the rest.
And news for you, China is not the West. It is a system of values by itself; a separate and distinct civilization. As a unified ideological bloc, the West, still having the technological and military upper hand, attempts to destabilize China by using various open an clandestine methods. It is a matter of willingness and capability and the West seems to have the former although its capability, albeit strong, is weakening.
It is only natural that the West, seeing China as a rival, will want to harm it. China, as the strongest developing nation, will develop reflections to thwart it. West hates China because China is China; not a copycat imitator of the Western value system. Hence there is an underlying war of systems, ideologies, models.
China is a developing nation, albeit a strong one, that is going through monumental drastic changes day and night as people's life is being improved and they are exposed to new stuff.
This is a delicate moment for China as it requires that the pace of development would not be compromised in any way. This is not necessarily because of a fear of internal instability, but because China does not want distraction.
Every system has some check in order to preserve system integrity, and, especially in the stage of fast development and dramatic change, excessive external influence might be destabilizing. This is only momentary, though. Once China achieves a developed status, builds its own strong national culture and entertainment industry, control the world opinion through an ever-penetrating international media industry, fully develops and theorizes its own indigenous political ideology as an alternative model, then China will go out compete the other alternative systems in all good will.