I read Starship Troopers and The Forever War back-to-back in like three days, and I wasn't so much of a fan of Haldeman's book, the writing just didn't seem as good to me as Heinlein's and in any case they're nearly identical -- though obviously the relativity thing and time making the "forever" part true was a very good point.
Saying they're "nearly identical" seems to miss the point that the former is a paen to the military filled with lieutenant colonels waxing philosophical about how duty to fight is the basis of all morality, whilst the latter involves a brazenly cynical protagonist whose thoughts of war mostly involve hoping to find a way out of it...
I always took that as a way to keep the economy and the war-machine separate. You get the general vibe that it's an otherwise post-scarcity society.
If you were a citizen, you were part of the part of society that used force. It's the basic Libertarian trope about force in that regard.
SFAIK, ST was based on ideas from the Swiss system, obviously drawn to the exponential.