I didn't see Marcus' throwing the idol in as idiotic.
The two who were salivating over it were Kenny and Crystal, and he managed to create a situation where no one was brave enough to admit they felt vulnerable.Marcus would not have been the one who got it for himself. Kenny and Charlie found the note, and Randy found the idol.
At the point Marcus threw it in the water, he was operating under the group agreement to set it free. Had he grabbed it and said Ha ha I'm keeping it, he would have lost all credibility.
Sure, if the buff draw were in any way foreseeable, he would have saved himself for one episode by reneging on the agreement to throw the idol in the sea, but he's not psychic.