The problem is disciplinary, not physical disability. So many of our youth have such defiant attitude problems that basic training would break them entirely. Some have watched leftist propaganda films like Full Metal Jacket and Platoon and they think that accurately sums up military service.
It does not. Only about 1/3 of our enlisted go to combat, and only about half of our officers ever see the heat of battle. The rest are "In the rear with the gear."
There are two kinds of recruits I don't want to see, however: leftist liberals, who take it on themselves to infiltrate the armed forces to demoralize the troops, and young men with a vendetta. Either presents a significant disciplinary problem: the first because he is such a coward that he will not fight, and the second because that trigger-happy SOB will open fire too soon.
Either way, these liabilities cost good men and women their lives.