They are in the army, though. As many have pointed out before in this thread, being in the army does not merely mean serving in Iraq or Afghanistan. There are a multitude of jobs in the army that require many enlisted and officers to stay in their homeland and serve in supportive duties.
It's the same with the navy. You don't have to be on a ship to be a sailor.
With the air force, you don't have to be flying planes to be an airman.
You don't have to be on the front lines to be in the military, period. No, not even when there is a war going on.
If William and Harry are just playing dress up, then all their fellow soldiers who are based at the same duty station where they work, well, all of them are playing dress up too, because by your argument, if you are not serving in Iraq or Afghanistan, you're not a real soldier, or really in the army.
No, no. The point isn't that the others are not soldiers. Of course, they are. But they could, at any moment, be shipped to any difficult designation, changing where they are today. Harry and William cannot. It is not their fault. But they cannot claim to be in the same status as those that could, which goes for the entire force, except them.