I suppose in USA you can get maybe arrested or pay some fine for cycling without a helmet, but in my country, it is still not a must. In bigger cities, like the capital, where the traffic is quite chaotic and pretty fast and constant, one is required to wear a helmet by the law, otherwise, they can pay some fine if they get caught.
In my little town, due to less commute on the streets, meaning less people,thus less traffic, one can ride his or her bike, without a helmet, and that is what you will usually see if you viist.
I wear my helmet sometimes, but mostly I don't, which is a big no-no, because helmets are there for a reason, to protect us from injuries. One can never predict if they are going to get hit by a car while cycling, or just fall off their bike. It can happen to everyone, everywhere. Thus, it is not really important if wearing a helmet is mandatory or optional, you have to bear in mind that your own personal safety is what matters here. If I rode my bike in the city, with a lot of traffic, I would never go without having a protection on my head.