The following is based upon Wikipedia. While it's not always 100% accurate, the references suggest that there's no reason to doubt the veracity of the information on this subject:
You need to remember that the vast majority of laws in the USA are made at state level, not by the Federal government. Seat belts are compulsory (for adults) in 49 of the 50 states but in 18 of those states drivers can't be 'ticketed' SOLELY because they've failed to wear a seat belt. (They can only have their failure to do so added onto a more serious motoring offence). Further, some states make wearing seat belts compulsory throughout the vehicle, whereas others only require belts to be used in front seats. In New Hampshire adults are exempt from having to wear seat belts:
http://en.wikipedia.o..._in_the_United_States
Chris