This is incorrect. USUHS grads are in fact guaranteed a slot as a military intern (PGY-1).
They are not guaranteed a slot as a resident - they need to apply along with all of the other interns for the residency (PGY-2+) that they want. If they are not selected, they are assigned duty as a GMO. At this point, they can either apply for residency training again, or serve our their obligation as a GMO and go for the civilian match.
I'm a USUHS grad (class of 2002), did a transitional internship 2002-2003, applied for but did not get anesthesia and ended up serving as a GMO with the Marines from 2003-2006. I applied again in 2005 for anesthesia and was selected, so I'll start my PGY-2 year this July.
The majority of Navy interns do not continue straight through from internship to residency; it's more common to do so in the Army or Air Force but not a sure thing by any stretch.
I saw several of my USUHS classmates on this year's Navy GME2+ seletion board result who were alternates. Going to USUHS does not guarantee you a residency position - the military can't force you into a specialty you don't want, but it can choose to employ you as a GMO if it fills its residency openings with people who are better qualified than you.
This is not to say that time spent as a GMO is all bad - I've enjoyed my tour with the Marines - just that anyone considering USUHS or HPSP should go in with their eyes wide open, understanding that there may be a break in their training that involves a trip to Iraq (or wherever).