Unfortunately, not wearing a seat belt isn't a risk borne just by the one in the seat. There have been cases of people flying out of their window and battering someone with their body due to not wearing a seat belt. Of course, everything carries a risk of harm to someone else, its a matter of where to draw the line.
We already legally force it in almost every state on public roads because it's not about you, it's about everybody else minding their own business getting killed by your choices.
Doesn't take much FOD on the highway for your unbuckled body to slam into something and now you have a driverless vehicle. Also every else in your car should be bucked too so you're not bumping noggins.
The same person who's super disciplined about seat belts likely takes other risks that another person would deem at least as serious.
Having an optional reminder feature is great; forcing it, not so much.