Bearing in mind that I married someone in the military and Daddy served in WWII I will say that you are correct to an extent. If someone is enough of a screw up, they figure out how to get rid of them. Fragging in Viet Nam, for example.
On the other hand there is solid reasoning behind the treatment new recruits receive in boot camp. War is no place for an individual and boot camp goes a (needed) long way towards eroding individuality. In the midst of a battle is no place for independent thought. If someone shouts "Get down!" there needs to be an unthinking immediate response. If a CO tells a soldier to "Drop trou and drop a deuce" the soldier needs to say "Sir, yes Sir! Where and how high?" Without that kind of response there is too much possibility that when someone shouts "Get down!" the soldier who hesitates even a fraction of a second can get his head blown off. Literally.
So, with that kind of background in mind with Drew, being told by a former CO and respected authority figure and having it backed up by a person who can at least portray gravitas and authority and Drew's general inexperience with relationships it makes sense that he would back down. Emotionally, as far as relationships go, Drew is possibly even less mature than the twins since he joined the military at 16.