Yup - and for the amounts of ammo you're ordering when you get $99 worth it's not overweight. Order 3 crates of 1500 7.62 x 39 and its probably overweight; 2 - 3 kg of .22 shells evidently not.
There's a difference between "rules" and "examples used to illustrate the potential application of rules".
Ammunition falls under dangerous goods (1.4) regardless of weight and according to THEIR shipping policy it is exempt from free shipping. Good on you if you managed to scam free shipping on ammo but that doesn't mean your statement about free shipping on the ammo is correct.
I don't really care one way or the other about it. I know that I was called once and informed that ammo was not covered by the free shipping so I cancelled the order (1 flat of 3" black cloud) . The policy is theirs and very much in place: the fact that a less than stellar employee decides to ship it anyway is a bonus for you I guess.