It's not just about the middle finger. It's that as well as the fact that he's doing it to a defeated, injured opponent. This is considered really, really dishonorable behaviour. If anyone has ever taken karate it probably makes more sense to them. When you hurt your opponent in a sparring session you're supposed to face the opposite direction and bow as a sign of respect for your injured opponent.
This kind of thing is a big deal in Japan. I'd be willing to bet that if he hadn't just broken his arm it wouldn't be as big of a deal, although it would still definitely be frowned upon.
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