I'm not sure why. Of the 3 Kool Stop salmon sets I've tried (thinline, dual-compound MTB, and full compound MTB), they've all squealed. Although when installed on my cantis, I find they don't squeal as much, if at all, so long as you keep both the rim surface (I clean mine with rubbing alcohol) and the salmon pads themselves clean (I sandpaper mine every week). When mine begin to squeel, I clean the rim and pads, then re-adjust the toe-in (I use a thick rubber band as a toe-in guide). This cures the squealing on my cantis under almost all conditions. Strangely enough, though, this exact same procedure failed to ever work on my V-brakes. It's interesting to note that the canti arms have almost no play when installed on the mounting posts, while the V-brakes (both stock Tektro and Avid SD5) are sloppy as hell.

But yeah, it's really baffling how the cheap no-brand pads don't squeal, while the supposed uber-quality Kool Stop salmons do.