Why is it against the rules to show a gun on social media? 
I do not understand what he did wrong.
Context, obviously
Yeah.  When the media is reporting on three thuggish incidents, don’t pull a gun out in a club, and then post it for the world to see.  It hurts the NBA’s brand, at the very least.
But, suspensions aren’t the worst thing that can happen to a kid that wants to live life like a gun-toting gangster.  I hope he’s got a few responsible voices around him.