A few ATS providers do allow leaves of absense precisely to allow their staff broader horizons - it's good for both parties. Norway's AVNOR for one does it.
Andanotherthing - I accept that you see loyalty from the services point of view and I respect that. Life in a blue or green suit demands that people can't just leave at any time, such as five minutes before deployment to an area in which bullets fly in uncomfortable directions. In civilian life, things are not so cut and dried. If you've given 8 or 10 years of your life to a company, leaving that company does not necessarily involve disloyalty. You may need to be in a different location for personal reasons or, like the young chap who started this thread, have a dream to chase. If a unit manager gives you his assurance that you're welcome back following such a move, you probably have a right to feel considerable disappointment if that assurance proves to be worthless.