If I recall correctly, the US military has annual training on these types of activities.....what you can do, what you can say, etc. when representing yourself as a military member.
Had this guy simply posted the comments on a FB page that didn't indicate any type of military affiliation (real or implied) he would still be serving.
His disclaimer doesn't invalidate those restrictions because he identified himself as a US Marine, and his page calls itself the "Armed Forces Tea Party Page".
Fact of the matter is this guy screwed up. He knew he could get in trouble, and if he claims otherwise - He's an idiot.