Yes.
The guy who put it up is an inconsiderate fool. In this country, you are free to be an inconsiderate fool. Which is lucky for you, because in this country you can even be the kind of fool who calls for spying on those that you disagree with.
I agree (obviously) that even in wartime, in this great country, you and I are free to be an inconsiderate fools. What this guy did went way beyond that - see his suspicious activity above re: lingering problems from Vietnam and what the NYT is doing to exploit them - I would doubt the Secret Service wants anyone unbalanced like him close to the President, for instance. I'm not calling for him to be spied on because of this sign - it's his behavior since then that raises red flags.