He's after money. Dozens of films have portrayed marines as war-loving nutcases before Avatar.
Besides, Avatar takes place in a fantasy world, it doesnt even try to be realistic. How can a RL marine be offended how marines are portrayed as in a fantasy movie?
Riiiiiiidiculous.
EDIT: Ok, I just finished reading the article and wow... just wow. Are we even sure he is a marine? Sounds like a cry-baby to me.