So, by your logic, it wouldn't matter if he brushed his teeth, either.
Health is a gift.
Throw it away and you don't get it back.
Many, many people enter the military and are not killed. They still have the rest of their lives to live. Their lives tend to be longer and less painful if they aren't addicted to substances that cause cancer, heart disease, emphysema and an assortment of other diseases.
If you expected your son to be killed, I guess it would be OK not to care about his long-term health.
The medic in the Marine's unit in Fallujah has been after him to quit, as the Marine himself admits in the article Tom linked to here. Maybe I should send the medic a care package. We idiots need to stick together.