MortalStrangelet

Justification varies depending on the agency. Public Health quarantines to isolate potential sick people and coordinate treatment of a suspected ailment. Law Enforcement corrals people that may have been involved in a crime.

I suppose the good news is that anthrax isn't contagious person-to-person, and even if you have a confirmed exposure 60 days of antibiotics will usually clear it right up. Also, the chances of the substance being anthrax is extremely small.

You are correct, quarantine isn't for sick people, it's for potentially sick people. Known carriers of specific contagious agents are put in isolation. Example: persons with active tuberculosis.