In countries where there's public healthcare, the burden of it on the public purse is a very real consideration where trade-offs must be made in varying ways. One of these ways is to tax things that contribute to higher loads on the spending of those public policies. Australia's tobacco laws are a key example of this.

To make it worse, people who don't wear a helmet aren't the best riders either and thus more likely to be involved in an accident.

A car driver has insurance to some level--a bicyclist or scooter user probably doesn't.

The issue is that in the US, we don't have universal healthcare which would make this discussion moot.