Because criticism of the people and institution of the Church was not considered in and of itself a heresy. Boccacio did not seek to undermine the theology of the Catholic Church, and the inquisition was only tasked with rooting out heresy. Yes, heresy could be liberally interpreted, but a tour through inquisition depositions reveals a tremendous amount of criticism of the Church and occasionally some pretty racy, sexy material, but that rarely had consequences. However, if one unrepentantly rejected the sacraments and teachings of the church, including the nature of Christ, etc., then one could be looking at penance and worse.