In fact, I believe it would be illegal to have the video visible to the driver.
Not true, you can buy plenty of aftermarket decks that will play DVD's in the dash.
Just because you can
do it does NOT make it legal.
They will but they can be bypassed very easily. My guess is that any place installing them will install them per directions including this parking brake switch. They don't wna tto get sued.
Most (all?) front DVD players install instructions tell you to connect it up to the parking brake so you cannot watch video unles the brake is applied.
"This unit detects whether your car's parking brake is engaged or no, and prevents you from watching a DVD or TV on this display while driving. When you attempt to watch a DVD or TV, the warning VIDEO VIEWING IS NOT AVAILABLE WHILE DRIVING
will appear on the screen.
To watch a DVD or TV on this display, park your car in a safe place and apply the parking brake."
"If the parking brake is not engaged, “DRIVER MUST NOT WATCH THE MONITOR WHILE DRIVING.” appears on the monitor, and no playback picture will be shown."
It's not difficult to disconnect the switch to the parking brake so it can be viewed while driving, but why???