An assist takes some skill. You have the ball. You must pass it to someone, avoiding the defense. If PA=0 you can't do this.
Catching the ball for a rebound doesn't take skill. You're standing somewhere on the court. The ball bounces off the rim and heads right for you. Even if the player on the other team has RB=20 and is perfect position, maybe it's a long rebound or crazy bounce that perfectly finds you. All you have to do is catch it.
I imagine if you have RB=0, HA=0 and ARMS=0 then you would have a problem catching the world's easiest rebound.