That's only part of it, US law requires safty systems to designed around for an adult not using a belt which results in a less safe car when someone wares a belt or is not an adult male.
PS: EU microcar are terrable in high speed crashes, but there not designed for high speed driving either. So it's generally not a real world issue.
Well thats some dumb regulation right there. There is a law that requires people to wear seatbelts and then you make another law that says design safety equipment as if people are ignoring the original law.
This penalises law abiding citizens to protect those who decide to ignore the law and not wear a seatbelt.
PS: EU microcar are terrable in high speed crashes, but there not designed for high speed driving either. So it's generally not a real world issue.