I know there is a lot of other things in life to get worked up about but to me it's about manners.
When my sister and I were growing up we always had to sit on the back of my mams car when another adult was in it.
I always sit in the back of the car when I am with my in laws so that one of them (usually FIL) can sit in the front.
The front passenger seat usually has more leg room, electric windows, access to the radio/music and air con/heating. People who say it doesn't matter which seat you sit in, would you sit in the back of the car if the front seat was free?
So when I'm getting picked up from work after a 10 hour shift in the car that I work hard to buy, tax, insure, mot, service and put petrol in and find one of the DC in the front then yes I shift them. Not because I'm being petty but because to me that's the good manners I expect from my kids, not just to me but to every adult who gets in the car.
It doesn't really matter in the grand scheme of life but then lots of things don't. Some people make their children wait to start eating until everyone is sat at the table. Some people don't let their children leave the table until everyone has finished their meal. I don't tell those people that it doesn't matter and they are being petty. It's just different things for different people.
It pissed the op off and I can see why. So can a lot of other posters.