A lot of people are making general statements, and I'm not sure how valid they are. For example, in my neck of the woods (Canada), I have flown without ID and without passing through security. I would be surprised if the same wasn't true in the US. What I left out: the flights weren't through an international airport and didn't connect to an international airport. Same airport, different flight (one that did connect to an international airport) and passing through security was a requirement. In that case, as well as domestic flights through international airports, ID checks were the domain of the airline.