I think you're confusing freier (free) with Ferien (holidays, originally holy days, but as in English now just holidays in the sense of vacation, though the idiom is confined largely to the various Schulferien, or school holidays). Since this sense wouldn't have existed in the 15th C. Ferjentsick can't have gotten their name from being men with free time or being free men. Ferien is related to Feier "celebration" and like English "fair" is from Latin Feriae, also related to festus and festival. It seems more appropriate to apply it to carnies than miners.