His being told by his superior officers to not engage in his political opinion is very near an illegal order in and of-itself. The Tea Party website holds a disclaimer that all posts are made by individuals and are not representation of opinion of the military as a whole. It doesn't matter that he sold anti-Obama bumper bumper stickers, that's voicing your opinion for the next candidate of the next election. The comment "As an active duty Marine, I say screw Obama" may be grounds for reprimand but being forcibly separated? PLEASE.
If you want to pull out the directive knock yourself out, I recommend starting with:
4.1.1.1. Register, vote, and express a personal opinion on political candidates and
issues, but not as a representative of the Armed Forces.
4.1.1.8. Display a political bumper sticker on the member’s private vehicle.
Comparing political dissent to contempt is a joke argument. The lines are shades of grey when it comes to speaking about the Presidency. Contempt would apply when it would come to one's direct chain-of-command not when it covers a political official, elected, into a public office. Everyone has a right to democracy even those who restricted their rights by joining the military.