The ESPN guys were suggesting the NBA was responsible for security. I do recall seeing Laker games in the past where they've had lots of those guys with yellow jackets with their backs to the floor the entire game, watching the stands.
I was at Game 6 of the ALCS this year, when some Yankee "fans" started throwing stuff on the field after a disputed call. The Yankee management had the riot police line up on the sidelines, and the cops went around hauling people off in the stands for throwing stuff, and the stadium was calm after that. I sure didn't see anybody hauled off tonight for throwing stuff, and the fact that fans could walk right onto the court after all this was frightening.
One thing New York City does, which other cities should copy, is that fans who go onto the field, or the court, face immediate arrest, and real charges.