The message of burning a flag is very clear, they hate America. As for my "silly Nazi reference" it's a comparison, not calling someone a Nazi. There are few things that are our of bounds socially now days. I could have said a Klan uniform but that's about it. The point is that some things that are legal can and should get you ostracized.
It could also mean America is in trouble because of burning itself up, metaphorically. (Would it make a difference if the flag being burned was also right side up or upside down?)