There seems to be some confusion here. The clerk offered you an anti-biotic for respiratory type infections. I don't like to randomly "treat" with anti-biotics as it is unknown if your chicks have anything by way of an infection. Further, the abuse and over use of anti-biotics leads to Super Bugs that out morph the antibiotics effectiveness leading to all sorts of problems for man and beast.
The "drug" or "medication" in most chick feed is just amprollium, a thiamine blocker, which prevents coccidosis from overwhelming the immune system of the chicks. That's all it does. Cocci is everywhere, being carried about by wild birds and such.
So to re-cap, some people feed medicated chick feed, while others do not. It's a choice. But just giving out anti-biotics to otherwise healthy chicks seems pointless to my mind.