Other phones have separate ports for power and audio out, so they can charge from the cigarette lighter with more power while still playing audio and GPS through the speakers.
Does sending audio through a cable really take that much power that a USB-compliant port won't also charge at the same time? Sounds like a problem with the car, not with the port. Just because it's running over the same cable doesn't mean it's automatically half the bandwidth.
Sending audio is not what consumes the power. The screen is on if you're using it for directions and the radio is constantly going on and to ping GPS and cell towers for location.
But that's not unique to the iPhone 7 or even to the iPhone. Literally any phone is going to need the screen on and ping the towers for GPS. Even if you're playing music over the headphone jack and charging from the USB port, you'll have the same issue. So what's the point?
The point is with the cigarette lighter you can choose an adapter that provides a sufficient amount of power. The cigarette outlet is rated for 5A at 12V which is plenty to amply charge the phone no matter what if you buy a decent adapter.
The current output of the USB port can vary from less than 1A to up to 2.4A at 5V which is not as much power. Some are so low that they're unable to charge the phone while its doing all those things. Since the iPhone 7 uses only one port for both power and audio/data out you're stuck with whatever the car manufacturer happened to provide in the USB port.
Let's face it, providing a robust USB port is not high on the priority list for car manufacturers. They probably have an intern throw that in as an afterthought and the primary purpose for it is transferring data, not power since every other device besides the iPhone has a separate port to provide power and doesn't usually rely on the USB.
