Then why should prowess with the rapier (for instance) be included in the Laurel, where prowess with rattan not be?

Because The Chivalry Are The Exception. That is, I accept (out of necessity) that the Chivalry simply don't fit in with any neat categorization: they occupy a weird and particular place in the SCA, and yes, they violate attempts to create a nicely consistent system. I simply throw up my hands at that, and try to work around it...