In North America, not wearing a seatbelt is against the law.
Literal
North-America [in-では] seatbelt [object-を] not-fasten [nominalizer-の] [topic-は] law [to-に] against.
法律に反する ('to be against the law') takes に for the law being violated. 反する ('to contradict, to be against') is a formal verb for opposition. シートベルト is standard katakana for the English loanword. 締める ('to tighten, to fasten') is the verb for buckling belts.