The other difference, and probably the most compelling: in this case, one could argue the Greig brought it on himself, or should have known it was coming etc., and he might have at least seen Reilly coming. In the other case, Turgeon scored a goal, he celebrated 'properly' (i.e he wasn't taunting the Caps players or making any sort of spectacle) and Hunter chased him down and blindsided him.
If the NHL didn't throw the book at Hunter, then basically it's open season any time a guy scores a goal and does anything other than shrug and skate quietly to the bench.