When do you ever have a chance to see the grip center on tv? They absolutely do not get sanded out, why would they? Ball rep lays out ball, scribes layout on ball, guy in the truck drills it, that's how it happens. On tour it's necessary.
In a regular pro shop, scribing a ball shouldn't be necessary. However, caring whether there are scribed lines on a ball or not is a lot of attention and focus wasted in the wrong area on the wrong things. Those are the same people who take a ball up to the front counter to complain when they get a couple scratches. Give the ego a rest.
I will say this though, if someone scribes, it says to me that they have a high attention to detail and are very confident in their abilities. No one is going to scratch lines on a ball, miss those lines, and then hand the ball across the counter to you. I feel it's unnecessary, I personally prefer not to, but wouldn't care whether mine were scribed or not. Never ceases to amaze me how picky and finicky and pissy bowlers can be over the most insignificant things.
I don't see the guys balls on tv that have scribe marks on them. So they do get sanded out. The guy above said the scribe mark stays in the ball.