I personally have not heard of any laws that say if the screen is visible to others outside the vehicle. Most OEM systems have 7 or 9 inch displays and I can tell you at night and from the back you can see them pretty well. My kids have watch videos in other peoples vans and SUV next to us on the highways.
I do know that many states have laws about the drivers being able to see the screen. Those laws say that and video display for the purpose of watching "movies" and the such must be behind the front seat.
Now there are some state who made watch porno in your car that others can see if illegal.