30% of that is federal though. You can argue that 11% tax rate on $500k is too high, but that's the only part coming from California.
Also, you'd still clear like $12k a month at 18 years old to play basketball. It makes sense why a kid would choose that route for a year, especially if he believes he's a top 5 pick either way or doesn't like school. Factor in food, gear, etc from the team and endorsement possibilities and it beats playing for free.
Now, you can get that tax free if you don't mind living in Baton Rouge.