Where does it say that? Read the decree from the Council of Trent again. It specifically states:
The Council of Trent in its decree "On Indulgences" (Sess. XXV) declares: "In granting indulgences the Council desires that moderation be observed in accordance with the ancient approved custom of the Church, lest through excessive ease ecclesiastical discipline be weakened; and further, seeking to correct the abuses that have crept in . . . it decrees that all criminal gain therewith connected shall be entirely done away with as a source of grievous abuse among the Christian people..
It speaks of granting (not selling) indulgences in moderation. Then it very clearly states that any and all criminal gain (i.e. selling indulgences) will be done away with.
If they are granting indulgences for nothing, than that is fine.
If they are *granting* indulgences after the exchange of or in exchange for, something, then it’s a sale.