I think there are a few possibilities:
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Because they're fucking insane.
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Because they're now cooperating for a plea deal.
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Because the photographer said, "Smile for the little birdie!!!" and they still had a Twitter logo next to the camera instead of an X because they thought that meant the mugshot wouldn't end up on the 'net.
Maybe I should read the article..
Edit: oops, it's "defiance" I guess. Maybe we can get another mugshot after their sentencing to see if they're still smiling In defiance.