Your argument amounts to "if you haven't experienced it, you don't have a valid opinion". By that logic, no one should ever comment on anything that they haven't experienced which makes absolutely zero sense, particularly in this case since everyone has a stake in how our military behaves.
I can understand your desire to defend these guys since you seemingly know what it's like to be them in some sense, but when they sign up to represent an entire country, then the members of that country can certainly have a valid opinion on their behavior. At the same time, non-military individuals should also seek to understand their potential mentality before making judgments which it seems most non-military people in this thread have done. There's really not more you can ask than to have people seek to understand before coming to conclusions. So unless the Marines stop representing me, unless you start paying my taxes and unless the actions of those soldiers stop having the potential to affect me, my family and my country, then that's what I'm going to do and your attempt to shout down any non-military opinion will be done in vain.