It's not strange at all. America is a federation of states which get to tax you, and get to pass their own laws. Your Mayor and Governor have much more impact in your day-to-day life than the President.
It just seems to strange to me they wouldn't be required in every state because it's such a severe safety hazard not to wear them.
When the US federal government has wanted to enforce things like a nation-wide speed limit, or vehicle safety requirements, they have had to threaten to withhold highway funding to get states to go along by passing state-by-state laws. In theory, the same could be done with helmet laws, but enough political energy has not materialized for that to happen. This pattern will have implications for full-self-driving vehicles.
Really in the weeds by now, but it just seems strange to me that each state has not decided, on it's own (as per the structure of our federal and state system of governance), to make helmets mandatory.