After decades of covering it up.
Note, I'm not American. My own country isn't squeaky clean either. No one's is. Hating on entire countries for what happened in the past is not a good move.
My grandfather was taken POW by the Japanese during WW2 and was forced to work on the infamous Thai-Burma railway. He suffered the after effects of that his entire life. I don't hate Japan despite having a personal connection to it.