I'm in no way in the military but..
I wouldn't be surprised if there's some "don't ask, don't tell" thing going on that no one is let in on. Steroids enhance performance. A soldier who uses steroids correctly has better performance than he would without them, and you can bet the military researched the shit out of that. Remember - they give amphetamines to pilots, they do drug up personnel with serious performance enhancing drugs.
If somebody handles their steroids well (no serious sides interfering with performance, no personality changes, no turning into the Hulk with nondrug grunts getting envious) its simply not in the military's interest to actively hunt down steroid users. Who carries the tripod, the mortar, loads the artillery shells better than someone giving it 110% or even 150%?
With Deca you are giving the military your best shot (lol), with heroin, no good will come of it.
They give you amphetamine when you are up and morphine when you are down - the military aren't pussies about drugs.
Its not the money. Most gear has long detection times and the military spends money like water. Its not in their best interest to actively persue steroid users who do it right.
If you do it wrong, and your gear gets in the way of your performance, the mission, decision making or morale, or with overt use or being caught with gear - then they might test and if they find you positive - their hand is forced.
Just my 2mg!