Sally
I run with a Lenser headtorch, and find it hard to imagine you're dazzling them.
I'm with Also-Ran and suspect it could just be bumps in the road that make it look as though the car drivers are flashing you.
Either way, you are perfectly in your rights to run with a headtorch to make sure you can see where you are putting your feet, and to make sure you're being seen by others, so don't worry about the etiquette...