I read the article. It says that it is legal to carry a firearm in all states, but some states require you to conceal your firearm.
Sorry to inject some common sense into US gun laws, but is this not simply ridiculous?
What if I was to openly carry a pen knife? Would I be breaking the law while a suicide bomber was perfectly legal because their weapon was concealed?
I suppose on some level it makes sense. If you're going to die, it might be best not to know, and if your job is law enforcement, it might be beneficial to argue that you didn't know the massacre was going to happen because the attack had concealed their weapon until the moment they used it.