That IS the good point. He broke no law because exact law said he can do this - and didn't have to have a reason. If what he did should be illegal, get the law changed. That is what elected representative government is about.
Can the government outlaw guns in public? Sure. That goes back to Sheriff's outlawing guns in open in town - while they could open carry out in the countryside. The right to have firearms is the right to have them, not the right to publicly display them or even have them concealed in public. That can be outlawed. To the contrary, that is specifically legal in that state.
While I favor open carry and conceal carry - us on opposition sides of the issue - and I don't think the 2A allows you to ban my having firearms, I completely agree the government would not violate 2A by prohibiting guns in public - displayed or concealed.
In this instance, specifically state law allowed him to do what he did - wisely or stupidly. Anyone may exercise a right - even if everyone hates it, fears it, and it is circumstantially unwise. Don't like that, change the law and/or what to get that person somehow anyway possible irregardless of his clear absolute and known legal rights?
Don't anyone understand the HUGE danger of allowing cops arresting people fully in compliance with law and prosecutors prosecuting someone for doing exactly what the law allows because they don't like that law or that person?
Why can't cops in NYC do stop and frisk? It's a good idea why should cops not just ignore Bill of Rights or Federal courts or law says? Well, the cops, prosecutors and courts should just ignore that. Right? ABSOLUTE NOT. Don't like a law, get it changed. Why can't cops ignore you demanding a lawyer or refusing to answer questions if the crime was really, really bad? Why can't cops beat you until to tell them where you hid the evidence? Why can't cops arrest protesters as terrorists for scaring people? The black marchers on the Selma bridge were "stupid" because they knew they might be attacked - and it upset the white folk - so certainly it was right to arrest and prosecute them? Then try to justify it by saying they were stupid and deliberately being troublemakers.
Don't like something in the Bill Of Rights or a law? Get it changed. Otherwise cops, prosecutors and courts - not just us peasants - have to go by the law and legal rights - like it or not. Cops and prosecutors can't just take away a statutory right claiming THEY decide what is best for everyone, not elected government or any rights a person has. That is called "police and prosecutorial abuse" - because that's what it is, even if you hate the guy and what he did.