MilleniaAntares is mostly correct, but there are some tangential reasons why Stalker operates differently as well. Each class is supposed to represent a slightly different "ideal" of the samurai when they take the Bushi template. The Warlord is the "Leader Samurai," the Warder is the "Protector Samurai," and the Stalker represents the "Killer Samurai." Because the first two classes' themes fit within the code of Bushido, they have more direct access to the Bushidos of the class, whereas the idea of a killer samurai was supposed to be something looked down on and so the Stalker has the freedom to not take Bushidos if it doesn't fit with the character.
So while the Bushi Warlord might best represent someone like
Oda Nobunaga, and the Bushi Warder might represent
Jin from "Samurai Champloo," the Bushi Stalker would be best at representing say
Ogami Itto from "Lone Wolf and Cub."