How is the 4th of July display described in the chapter an instance of irony?

Even the setting up of the chargers reminded Norman of setting up guns in the war, "Along an unused railway spur, four workmen labored in the shadowy red heat, setting up a platform and steel launchers for the evening fireworks."

Norman muses that,

"Later they would blow color into the sky. The lake would sparkle with reds and blues and greens, like a mirror, and the picnickers would make low sounds of appreciation."

Norman remembers the fireworks that exploded over Song Tra Bong but they were bombs and nobody, not any side, really appreciated them.