Think all that stuff on the screen (curb, grass, holes, trash cans, etc.) are there for decoration? Hardly! Dribbling on these surfaces can cause your player to lose control of the ball!
Not every court has well-defined out of bounds areas. Just like in real life pickup games, the rules aren't set in stone. :) For example, on the city court, if the ball hits to doors or trash cans, it's not out! It'll bounce back onto the court and any player can pick it up.