if you spray the weed, it doesn't just disappear, it goes back to the ground.
also, if you spray your whole yard where the chickens have access, they will eat the grass and weeds that you just sprayed - eating the weed killer
i don't have anything against spraying your yard or using weed and feed; i use them. i don't let my chickens in that part of the yard, though. their part of the yard has weeds, and i don't treat that area (they've got a large area of grass, plus an acre and a half of new pasture, they don't make the trek to the front of the house.
also, the grass that doesn't die, as it won't kill the grass, will contain residual. for instance, the bifen IT i spray on my house for bees, ants and spider has a residual period of 3 months. it is encapsulated and releases over a long period.
some herbicides do the same. so, by letting them eat the grass, you'd be letting them eat the herbicide