"How about a t-shirt that says "I support our troops when they shoot their officers?" Is that protected speech?"
Not the situation described. Personally, I have never encountered that t-shirt. I'd certainly have words to say to any creep who was wearing one anywhere near me. However, since that shirt does not represent a threat to me or to any class I belong to, I would probably keep it at words.
It was a now famous photo from a leftist protest of a few years ago. But several protesters made their own with white tshirts and sharpies and wore them into the freedom walk at the Pentagon last Sunday and were not forced to leave. Incredibly insulting, and IMO, fighting words. But, our side respects others rights which has gotten us into the sorry state we find ourselves in -- staring communism in the face in our own country.