Here in Ft Lauderdale I cannot use a public library computer without entering my library card number. Two years ago things were looser. Some of the smaller older libraries here *might* let you get on line just by signing in
I live in Oklahoma City and the public library here just lets you sign on, no id or library card number. They do limit the time to 30 minutes and you can sign up and reserve a time slot. So, if I was a terrorist and I used a public computer at a public library here in OKC, there would be no way to track the use of that particular computer back to me.