Seatbelts shouldn't be mandated. People should have the choice, you just have to make the case that it's worth it. In fact some places don't mandate seatbelts (New Hampshire doesn't, aligning with their motto "Live Free or Die").

That said, for those who cannot advocate for themselves, like children, I think it's entirely reasonable to hold someone accountable if they decide to forgo an appropriate safety mechanism that results in harm.