I think it is fair enough.
Comparing it to people who take risks through extreme sports, certain life style choices etc just doesn't work, it would be impossible to draw the line in those scenarios, what would be an acceptable level of unhealthy or risky life choices and what wouldn't? Hospitals are not at capacity with people falling off trees or in skiing accidents, they are at capacity with (unvaccinated) covid. Yes there are people who in hospital from drug use, alcoholism and poor diet but there are often complex socio economic, culture, education/ awareness issues and psychological issues surrounding this, so again impossible to draw a line.
To be vaccinated and to not be vaccinated is very black and white, it isnt subjective and the line is clearly there to determine inclusion or exclusion for free covid care.
Also, as stated before Singapore does not have a completely free health care system, they made a covid exception. If you choose not to get vaccinated you choose to take the risk of having more chances of getting seriously Ill and not having free treatment should that happen.