> How is that different from cars that project information on the windshield?

Are there any cars like that?

At any rate, for such cars to receive regulatory approval, they would need to be vetted to ensure that the UI elements don't exacerbate blind spots or hide critical pieces of the road, etc. The Apple UI is not designed with that in mind and may not give any such guarantees, and even if it were, it's still possible to abuse. There is also a formal process for removing such cars from the road if it turns out that they don't function as expected; whereas chasing down Apple Pro Vision users is left to the relevant highway patrol units.

Even if, in theory, the Apple device could be used safely, what happens in practice is far more important than theoretical use cases.