Well of course it's used, it has 13,000 miles on it But as dfelt pointed out, it was originally a private sale which is to say from the pictures that it looks like it has never seen police duty (you can tell that it's a 9C3 detective car, but is still too nice of an interior - factory console, cloth rear seats - to have been used in police services).
And when they first hit the dealer's lots some had the sense to spec them out for civilian sales, and got away with it, before GM learned of this news and put an end to it (I think there's an AutoBlog or Jalopnik article to verify this statement).
I agree with your first statement, as this dealer is offering it through the Chevrolet Certified Pre-Owned program (I'm not positive fleet vehicles qualify for this program, especially used police vehicles). It seems as if he's found a loop hole for this particular unit (maybe because of it's private sale status and not fleet status). I know that the Caprice(LWB) model doesn't offer a wagon, but I'd be just as happy with the Commordore's wagon as I would with this LWB Caprice
Still, GM/Chevrolet should have offered the Caprice as the full-size model and put the current Impala to fleet sales only (a'la Captiva). After driving the '13 Ford Taurus, I feel the Impala is a joke (if in the market for a large car today, I wouldn't even consider the Impala and would go right to the LaCrosse).