Surely its the other way around. It's reasonable to expect all adults to wear a seatbelt in a taxi but not reasonable to expect all taxis to carry child seats. It wouldn't be unusual to expect on occasion for a parent to travel with 3-3 children; should all taxis therefore carry 3 child seats? No. Should all adults travelling in taxis use a seatbelt when one is available and required when travelling in private transport? Surely yes.