Closest I can get to a rational explanation is anecdotal.
My wife drives a convertible car. If I drive that with the hood down, I believe I notice a significant change in behaviour from other road users than if I drive the same car with the hood up, they are much more courteous - often behaving completely unexpectedly. Stuck in a wrong lane etc, they'll make gaps, won't tailgate, It's not because I look intimidating (not since I got the face and neck tattoos lasered /s) I believe it's because they identify the car as being something with a fellow human being in it, rather than an obstacle in their way, and/or are more aware of "personal space" instinct.
Could be wrong - but perhaps the observation above about clearance distances is from "personal space" consideration, rather than or as well as risk compensation.
Helmets are somewhat depersonalising - even if not for covering up hair (or a bald pate), then for automatic assumption that it's a "%^$ing cyclist in the way" - rather than another human being.
Interesting. I think a lot of people do defer to a nice car on the road. At least those that appreciate nice cars or don't want to have to pay the bill for putting a dent in one. If I see your convertible I actually might think, "that's a tax payer, I'll be nice".
I can't say I've noticed any emotional response on seeing a helmet less cyclist other than "jeez look at that clown, he doesn't want to live long".
It's a 1998 MX5, worth perhaps $5k - about the price of a single Range Rover headlight I guess. To put it another way, I usually drove a big ugly black truck which I've traded in for a nice new expensive shiney one, and I'm absolutely positive that drivers give me a hell of a lot less slack in the shiney new truck which would cost them dearly if they hit it, than the old one which looked like it was built like a tank and they'd have been lucky to damage.
So if you really thought a cyclist without a helmet was a clown who "doesn't want to live long" because he put himself in danger - I do think you'd compensate in your behaviour (even if unconsciously) giving him a bit of slack perhaps - unless we've all become so callous, selfish, and judgemental. I think that most likely you're a good person - so would in fact have a socially appropriate emotional and behavioural response.