The reason it's unwise to feed lawn clippings to horses or goats or whatnot is that a) if there is any delay at all between the cutting and the eating, they can start to heat up and ferment, which can do Bad Things to the animal's digestion; and b) they can pack into a choking mass in the animal's throat, because of their consistency.
I should not think that SMALL amounts of FRESH or WELL DRIED (nothing in between) lawn clippings should pose a threat to chickens, and mine in the tractor certainly eat whatever gets flung their way by our rather erratic mulching mower. I sure wouldn't dump a great bucket of 'em at the chickens, though... as with so many things in life, you might get away with it for a while but why take the risk.
JMO,
Pat