Certain leagues in the USA have helmet restrictions. This is primarily for leagues which rent the ground from county or state. County baseball fields are the closest ones to cricket and these grounds have insurance requirements. I have heard of one such case in Carlsbad county - u can't bat in a softball game without a helmet ! The team will be issued a penalty (fine of some $$$) by the ground staff and the organizer of the league will be penalized for not enforcing the regulations. Pretty ridiculous, but sometimes leagues have to agree to certain stupid state regulations so that we can continue playing cricket.
I play my T20 games there and have worn my helmet always. For 40 over games, i have played near Los Angeles, they have proper grounds with proper wickets. No rule is enforced on wearing helmets though.