Two points from Legend website:-
Legend’s touring specific coil spring rates provide superior ride comfort, improved performance, and increased resistance to bottoming for all day comfort
Available in 12″ and 13″, standard or heavy-duty applications

The shocks I have bought that had standard springs didn't bottom out either as the spring rate was too high for my weight and the stock HD shocks were also not suited to my weight. It is not hard to make a shock that won't bottom out but it could well be too firm for any real comfort, IMO.

 

A lot going on here but basically it's just "single rate spring vs progressive rate spring"
Notice how much the progressive rate spring drops the wheel in that big dip. This would prevent the rear wheel from instantaneously unloading and kicking out in a hard panic stop on a Sportster like they all do!.. Because Harley has only ever built them with a single rate spring as far as I know.