Hard to say. There are all kinds of laws that aren't criminal statutes. It seems to be as "illegal" as not observing a moment of silence for fallen veterans on Monday.

Perhaps I'm a tool for saying so, but I appreciate the existence of a uniform code for respecting the symbols of the United States. Gives people, local governments, and the military understandable guidelines for showing patriotic unity, and it gives protesters a clear standard against which to rebel.