Why can't American football fans just be nice like rugby ones?

Here's two contrasting photos, one shows a typical after game riot in America where fans went on the rampage after the superbowl. The other shows a typical after match seen between European rugby fans. Why can't Americans be nice like us?
Why cant American football fans just be nice like rugby ones?
Why cant American football fans just be nice like rugby ones?
Why can't American football fans just be nice like rugby ones?
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