Why?
Dymondwood isn't anymore prone to crushing than fiberglass and there's more material on the Grizzly (and better shaped) than on the SuperK and T/D. If anything the tips on the Grizz are better suited for a low-stretch string.
The only reasoning I can think of is Bear wants to sell their higher-end bows. Specifically say the bow comes with dacron and guys will be more likely to say, "Well gee, maybe I'll spend the extra bit and get the SuperK."
As I said, mine came with FF and the replacement Bear string was also FF; I see no reason the dymondwood overlays aren't FF capable, especially considering that's what other bowyers are using.
YMMV, but the newer Grizz shoots a whole lot better with a low-stretch string (and even better if you reduce the tip width and mass to something more reasonable. There's no way I'm putting a low-stretch string on my '59 Grizz though
edit: corrected KS to SuperK... guess I'm stuck in the 50's :D