On May 15th a ship dredging the site of the attack on a South Korean warship in March that killed 46 seamen made a spectacular find: propellers, motors and a steering section that international investigators say "perfectly match" those of a CHT-02D torpedo that North Korea sells abroad. What's more, the blue marking was similar to one on a previously captured North Korean torpedo. This was as close to a smoking gun as the South Koreans could have hoped to find.
The discovery, combined with intelligence reports indicating North Korean submarines were out of port during the attack, allowed the investigators to conclude on May 20th that the Cheonan "was sunk as the result of an external underwater explosion caused by a torpedo made in North Korea."
Indeed, despite a national outpouring of grief, the senseless attack aroused surprisingly few public demonstrations of wrath with the North. Brian Myers, a writer on North Korea, notes that there was more palpable anger in 2002 when an American army vehicle ran over two South Korean schoolgirls.
from this article
So basically, an accidental killing of two people generates more anger and protest than a deliberate killing of 46. Why might this be? There are several possibilities:
1) The people of South Korea think that the lives of two schoolgirls are worth more than the lives of 46 sailors.
2) The people of South Korea expect bad behavior from North Korea and are not surprised by it, so there is no emotional impact, while they are shocked by anything bad the USA is responsible for.
3) The people of South Korea calculate that public protest is much more likely to change the actions of the USA and prevent future tragedies.
4) The people of South Korea are inherently racist and/or anti-American and therefore look for an excuse to bash us.
My initial reaction was to assume #2, but after considering it further I think that #3 is the best answer. It makes sense to protest things that your protest actually has a chance of affecting. Therefore, while it seems unfair to protest small things while ignoring big things, it is actually rational, because protesting a small thing that you can have a real impact on improves the world more than protesting a big important thing that you cannot change at all.
But it may also be true that you can get away with more things if people expect no better from you. The USA, and developed Western European countries, are held to a higher standard of behavior than most other countries. Rather than complain about this, we should be proud of it.