He's a master sergeant, once of the highest enlisted ranks, involved in planning the mission. Not low rank at all.

A lot of people were involved with the mission that were not themselves directly endangered but could still stand to profit.

Easy to think of a scenario where he should stand down but instead continues with the mission, risking the lives of his team mates, or maybe civilians.

He was incentivized to win the bet. The military does not want people to have outside loyalties because that creates problems any time they’re not perfectly aligned – for example, if they had orders to minimize team or civilian casualties you don’t want this guy starting a messy firefight because he’s thinking the target is getting away and willing to risk someone else’s lives for half a million dollars.

You also have to think about leaning information: if people do this, bodyguards around the world are going to monitor betting markets looking for unexplained changes. The military doesn’t like anything which can leak timing information since that increases the risk of a mission failing.