I think overall US has safer regulations, but either way, they are different. I'm sure a lot of them are the same for both markets, just not necessarily tested/approved for both.
I know (well I think) in addition to all of the actual requirements for seats, there is something about how emergency people must be able to remove them easily. I used to have a 1993 Dodge Intrepid with the build in child seat. We loved it, my son was 2½ when we got it, and it was super roomy and nice, and when folded up, the seat looked nicer than the ones without the built in baby seat. They also offered it in minivans. I think a lot of the reason they stopped that was in a wreck, they had to remove the children from the seat, but they prefer to leave them in it if possible. Which is why Volvo now had built in "Boosters" not actual baby seats.