I suppose I could go shopping for children's car seats, but it's easier to ask youse guys.
How old does a kid half to be before he's able to unbuckle himself from a car seat?
Does the typical seat just have a strap or two in front of the kids chest so when he's strong enough to squeeze the "tabs" in, he's strong enough to get out?
And are their seats designed to make this impossible for someone who can get out of other seats? For example the latch could be put where he can't reach it.
I can see a reason for both kinds. While I wouldn't do it for long periods or without good reason, when the kid and one parent are out and about, you don't want a kid unbuckling himself in the back seat out of view of the driver (and even another passenger) and then what, opening the door and falling out? Or at least wandering away.
But OTOH I see repeated stories of parents passed out (or dead?) in the front seat while the kid is strapped in in the back. What if it's a place where no one finds them? If the kid could get out of the seat, if he's old enough to do so, and get out of the car, he stands a better chance. It would be like enemy torture to spend days imprisoned in the seat until one dies.