The original fear of mandating seatbelts was it becoming a slippery slope, and the government continuing to mandate other aspects of citizens lives.

A similar fear happened when drunk driving was outlawed, but obviously its implications in harming others was a good justification for it.

With seatbelts, it’s less harm on others if I don’t wear it, more so a strain on society as a whole (first responders, more serious medical attention)

In general though I agree that governments shouldn’t be mandating what individuals can do to themselves. The argument lies in how much those actions effect others in a tertiary sense (doing drugs only effects me, but if I go into a coma that’s a strain on society and a blurry line. If become violent because of those drugs, it’s more concrete)

Meanwhile alcohol is legal, and is involved in more murders and domestic violence than any other substance.