Why wasn't Ron Artest arrested for assault?
He still might be. Detroit prosecutors are reviewing all the tape of the event to see if they can make a felony battery charge stand up in court. They have to be on solid ground because Artest is a millionaire, and he can hire as many lawyers as he needs, who can spend as much time and money as they need to to, for example, get the videotape of the incident ruled inadmissible because of some arcane property rule. The district attorney's office doesn't want to be embarrassed by bringing charges and having them dismissed. Two laws in this world, one for us, one for the rich.
The real reason is because Ron Artest was assaulted first. Each fan that was attacked either threw cups of water, threw a punch, or walked up aggressively in the middle of a brawl to the players. They were consenting to a fight. If you throw water at somebody, you're consenting to a punch in the face, and you can't try to screw the person you attacked with legal charges.