Earlier quoted context omitted.
You make it sound like it is perfectly ethical and reasonable to interfere with another sovereign state's defense systems unilaterally. Edit: I think I have read much into the parent comment and made a strawman fallacy here.
I guess you didn't see my comment below that China deployed long range radar system to monitor the sky of South Korea and Japan like 10 years ago? Or are you just ignoring that? 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.