You need to think of this as a UI guideline, not a gun pointed at someone's face. A quasi-standard, not a regulation (even though it might be coming from regulators).
If done correctly, a user will select the best mode, not to save their life, but to maximize their own convenience. People do want to interact with their device when they're driving, and this isn't even a mistake. The problem is that the best UI when you're not driving, is a horrible UI when you are driving, and probably vice-versa.
Depending on how software authors adopt the setting, it might be:
Voice control when driving, otherwise stop listening and making incorrect inferences when I'm not driving.
Display to HUD when driving, display to screen when not.
STFU about trivial nonsense notifications when driving. Bombard me with a bunch of shit that I'm finally capable of handling now, when not driving.
"Blocking" things doesn't necessarily mean it's something you do to the user; it's something you do for the user because they've requested it as a matter of convenience. That's the key to doing this right.