Many antibiotics are effective for a variety of diseases, but there is not one antibiotic that is effective against all diseases. Thus, not a single pill that can protect against all types of biological weapon attacks. Keeping a supply of antibiotics on hand poses other problems because the antibiotics have a limited shelf life before they lose their strength.
There is currently not any justification for taking antibiotics as a generic protection. Also, it should be known that antibiotics can cause side effects. Using antibiotics when not necessarily will eventually make them not effective against the infectious agents they are used to fight, by promoting the rise of resistant strains.