The comment suggests taking actions against possible nuclear development, while China's position has always been "no nuclear on Korean peninsula". We know that the statement or stand had no real effects. So the only other possible alternative would be to interfere directly to stop them from developing their defense systems. So I thought the parent comment is suggesting that.
I phrased it in such a way to draw parallel with US military actions recently to show how that is equally undesirable.
US military is present in South Korea and Japan because of N. Korea and China, not anybody else.
You don't see such US military presence in NZ or Australia.
I'm not sure that paints an accurate picture.
The first reason the U.S. is in Japan and S Korea is because the U.S. won WWII and setup bases in captured territory. If you look at most major U.S. bases around the world, you will notice they are in territory captured in WWII: Germany, Italy, Japan, S Korea, the Philippines (legal and future status uncertain), etc.
It's the same reason the UK controls Gibralter (since a war in ~1704!), Russia controls Kaliningrad (WWII), Taiwan is relatively independent (Japan conquered it in the late 19th century, then the U.S. took it in WWII), Hong Kong (UK in 19th century), etc. It's the spoils of war, and the consequences can last perpetually - until the next major war. Imagine how different China's situation would be if those bases weren't there.
The second reason is that East Asia is militarily unstable, and the U.S. presence provides stability appreciated by all. Otherwise all these very distrustful neighbors would build up militaries, disrupt trade, and end up in wars. China may be the exception, but probably even they don't want the U.S. to just walk away, leave a vacuum of chaos, destabilize the Korean Peninsula, and compel Japan to becoming a full military power again; they probably want to slowly replace U.S. power with their own.
Australia and New Zealand are not militarily unstable, though they do have close relationships with U.S. military and security institutions. The U.S. does base troops in N. Australia now, to support security to the north.
I think you supported by argument. Why is East Asia militarily unstable?
Some is basic international relations (which I'm no expert in, and I'm writing quickly off the top of my head ...):
Fundamentally, there are multiple powers in close proximity (China, Japan, Russia, and currently S Korea) who are militarily competitive (i.e., there's not one clearly dominant power, as in N. America, whom the others can't challenge) and vulnerable to each other (e.g., not separated by an ocean, like the U.S. and other powers). This is a sure recipe for wars, and there's a history of it happening in East Asia. Furthermore, there is a lot of nationalistic hatred, especially between Japan and the other powers. Another example of powers in proximity is Europe through WWII, where wars were fought for hundreds of years.
Domestically, conflicts are resolved through rules (laws) and mechanisms (courts), and there is an authority to enforce it all (government). In the international arena there is no authority; it's fundamentally Darwinian anarchy and nations, fearing for their very survival or seeing opportunity in the lawlessness, end up shooting at each other. The way Europe stopped having wars was to create the European Union, which provided the authority, rules, and mechanisms (and the American presence helped too). Globally, the United Nations and "international law" serve that purpose to a degree, and international wars have been greatly reduced since WWII, but really there is no authority with enforcement power.
But there is, to an extent, one exception to that last sentence, the United States. The U.S. can play that role to a degree. They aren't a legitimate authority, in that nobody elected them and they represent nobody but their own interests (hopefully enlightened), but they have the power to enforce things when they want to. Much of the world, desiring some order and wishing to avoid crippling wars, supports it to a degree. The U.S. is, relatively to predecessors and to potential alternatives (China, the old USSR, etc.), a responsible steward: They want peace, human rights, and prosperity. The U.S. is averse to conquering foreign lands (i.e., to keep them) and long-term wars. When you hear American officials talk about maintaining the 'rules-based international order' (referred to by some others as the 'U.S.-led international order'), this role is what they mean.
That's the role the U.S. plays in East Asia. They have the power to keep a lid on conflict there, and therefore the local powers don't need to build up militarily and to seek every advantage to ensure their survival.
China is now challenging the U.S.-led international order, including many of the rules that they feel don't suit them. The more the East Asian powers see the U.S. hesitate to fulfill its role, such as during the recent Presidential campaign, the more they build up their own militaries (which is happening rapidly) or consider making a deal with China (which is discussed, even in Australia).
So even if US guarantees nuclear umbrella for SKorea and Japan, you never know what will happen. And you don't want to take chances with N Korea there.
Japan's recent military buildup didn't begin until China's really took flight. Japan always has faced little threat from N Korea (besides the nuclear weapons); NK would have to cross a significant body of water to reach Japan, and lacks the capability, and Japan has more than enough conventional power to easily defeat NK. To deal with NK, Japan needs only anti-missile and anti-air capabilities to protect against nuclear weapons.
S Korea obviously is much more exposed to NK, but AFAIK their risk is not defeat to NK but losing very large numbers of civilians in Seoul, which the North holds hostage with massive, dug-in artillery batteries. That has been the North's deterrent against an SK invasion. I'm not sure if SK's military is growing.
I was referring to the "possible actions" that could have been taken against NK by China earlier, proposed by the parent comment.
On the other hand, your point would apply pretty well to China's response to South Korea's defense systems. (It's not just that China's calling it espionage; the article mentions serious hacking attacks.)
Basically on this matter, the Chinese government has taken a similar approach as the previous US administration, which is to not take actions. On hindsight of course it is easy to say China (Obama) should have done this or that. But it was quite hard to justify actions against NK until pretty recently. And indeed actions were taken in the form of economic sanctions in light of recent events.
The US have an embargo against North Korea, whereas the chinese are selling them materials to build their missiles with.
They could start by putting political pressure on them, levying trade sanctions and publicly denouncing their nuclear program.
North Korea’s Hydrogen Bomb Claim Strains Ties With China
JAN. 6, 2016
https://www.nytimes.com/2016/01/07/world/asia/north-korea-ch...
China announces sanctions against North Korea
5 April 2016
http://www.independent.co.uk/news/world/asia/china-announces...