a show of force persues one objective, to show the enemy what they are up against and deter them from atacking. do you think its better to do nothing?
"Hey they just shelled us, but lets do nothing, those mongrels will implode one of these years, yes yes!"
what the NK did wasnt a show of force, it was an attack, an act of war.
The south koreans arent interested in pissing off the north, all they want is to continue to prosper as a nation as they have done for the past 50 years. they wouldnt risk all the gained for a pity conflict with the north.