Few would touch 99% of freedoms, or freedoms that have even trivial benefit. Like free speech, obviously. But what about stealing? You say that isn't a freedom. It is. It's a freedom that should clearly be restricted.
How do people who don't wear their motorcycle helmets victimize us? Well, to use cold conservative logic, they cause economic damage because it takes more time to clear out the dead body from the street. The police will come, there will likely be courts brought in, any damages and perturbances to the economy will be far more pronounced, whenever someone dies. A funeral will have to be held, if the man has little money, it will be held at state expense, unless libertarians are now into dumping the bodies of poor people into the oceans. This will far exceed the costs of any treatment we'd need to perform on a person who had injuries with death that was prevented because of a helmet.
Also, they steal from their mothers and the family the water that drains out of their tear ducts. Libertarians don't count emotional damage into their vastly simplified equation of life. But it does matter something big. The biggest cost of death isn't nearly just the economic ones.
And what do we get out of all this? The freedom to not wear helmets? A huge loss, for a nothing return.