Is it public information to find out why a person is in jail?

It is public information indeed to know why a person is in jail but it is not always so simple as to get it from a directory that is something you simply log onto. You can check out your court house and see if you can order a copy of the court order that placed this person in custody after conviction. You can contact the prosecutor's office and ask for a copy of the public order. You can do some internet searches yourself and see if that information is available to you elsewhere. Now, if this person has only been arrested and charges are still pending, you may be able to order a copy of the arrest report (redacted possibly to not disclose private information) from the arresting officer's commanding officer. You do this by going to the police station and asking for a copy. Do some research on your state's version of the Freedom of Information Act (unless the jail is a federal one, then the Freedom of Information Act would automatically apply). That statute can help you determine what to ask for and how to ask for it and you can verify if it specific prohibits you from obtaining certain arrest records or police reports.