The big problem with letting the user decide to do something like this is that distracted driving doesn’t affect just you, but everyone around you. There’s a reason drunk driving is illegal: you’re a danger to others on the road. Hence why being a drunk passenger is fine; you’re not a danger to the other drivers on the road.
So, should Apple be restricting what the user can do with their device? Depends on who you ask. But when it comes to distracted driving, it’s pretty well agreed that it’s wrong.
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Now, if you still want to watch YouTube while driving, you actually can. Just load up the YouTube app on your phone and hit play. They don’t stop you. Apple just won’t let you put it on your car’s screen.