While I understand that the march was intended to be apolitical, in reality you can't support veterans without implicitly supporting American militarism. As you say, they "had to do violence somewhere"--violence in the name of U.S. imperialism. (There are small exceptions to this--the Civil War, for example, and WWII, to a certain extent)

This is not to vilify veterans or encourage the type of behavior displayed toward them during Vietnam; we should respect all people. But respect does not have to equal approval or honor.