I have something I'm really curious about... Is there some kind of secret agreement between South Korea and North Korea?

Not long ago, Oh Jong-taek and several others were arrested and punished
for sending drones to North Korea....
But when North Korea sends drones to us or lays landmines
causing casualties, why can't our government protest?
Is there perhaps some kind of secret agreement between the government and North Korea?
I am truly curious.