Why does the gemara only use the phrase pattur aval assur in reference to laws of shabbat? Regarding laws of other holidays, the gemara says assur from the torah or assur from the rabanan. Why then did the gemara only by shabbat discussions decide to use chayav to refer to assur from the torah, and assur to refer to assur from the rabanan?
Note that sorcery is the only other topic in the gemara that also uses the terminology of pattur aval assur.
Patur aval asur means that a person is not allowed to do the action but is not required to bring a korbon chatas if not done deliberately nor chayav kareis if done deliberately. Since one cannot be brought before the court, the term is used specifically for actions of this type. If a person is able to be brought before a court then the other terms are used.