Communicating in Cars

Overheard in the car on the way to school this morning:

Speedy: Like, they had to do an essay on How to Kill a Mockingbird.
Dreamer: (deadpan) Wouldn't you just strangle it?

(Note: Speedy has not read To Kill a Mockingbird, and has mis-heard the title. Dreamer was being his usual literal self. He is getting much better at recognising when he is being literal, and turning it into a joke. Speedy has been working on improving his marks in English with a view to moving to the advanced class. He needs to get A's and is almost there-. He was telling me why he has changed his mind about doing advanced English).

I shouldn't really drive Speedy and Dreamer to school. They have legs that work. They could do with the exercise. School is only about 1.5kms away.

Dreamer, though, absolutely insists on carrying everything he might possibly, even conceivably need - a folder with a notebook for each class, previous notebook for each class just in case he needs to refer back to something they studied last term, text book for each class (whopping big Physics, Chemistry, Maths texts), lunch box, drink bottle, at least two novels, and a Nintendo DS.

I've tried. I've tried to the n-th degree, where n = ASD. "Dreamy, do you really need two novels? Wouldn't one be enough?".
"But Mum, I might finish one, and then I'd need a spare"
"You're only half way through the first one. You won't finish that today. Leave one at home. Please?"

And so, his backpack weighs in daily at around 15kgs, and I take pity and make a deal. I drive them to school, and they walk home.

They say that taxi drivers and hairdressers hear more secrets. Mum's taxi drivers do too. I'm delighted to say that my ASD son becomes positively verbose in the back seat of a car.

I can ask leading questions, and actually get thoughtful answers instead of the usual "dunno" or "maybe". If there are two or more kids in the car, I can just keep my mouth shut and eavesdrop. If I want to find out what's really going on in their lives, how they are feeling, or what's upsetting them, all I have to do is offer to drive them somewhere.

Why don't they realise I am there?  Why aren't the usual rules of censoring disclosure in conversation applied? Does it have anything to do with the speaker not facing the listener?

Is it like an ostrich putting it's head in the sand to avoid enemies- if you can't see someone's face, then you aren't talking to them, and they can't hear you?