The legalities would include the GPS or video while driving. Same reason why US cars cannot be started without the clutch being pushed in. Most people have neough common sense to only start a car when in neutral, or when the clutch is in. Duh. But the few dum-dums who didn't and sued the auto manufacturers in states are the ones who rendered that no good. Being me, I have disabled this "Safety feature" too on my car because I will yell at myself if I happen to start the car in gear, not blame the manufacturer for my mistake.
I donate a looney.
I think it all comes down to this. If you like the configuration and building aspect of a CarPC, then StreetDeck == BAD. If you like to reap the benefits of the configuration and building yet find no interest in that aspect, then StreetDeck == GOOD.