Probably true. But I think a bigger reason is b/c blackies tend to be more timid and easier to scare away...yelling at them or blowing a whistle is often enough. Not so with a griz.
Either way, hammockers are in no greater danger than someone in a tent. It's very unlikely that any species of bear would bother a hammocker unless they've become habituated or you cooked near your hammock in griz country...JMHO. But then, if you look hard enough for a bogeyman, you can always find enough stats to convince yourself he's out there.