I imagine that was part of Christ's zeal for His Father's house.
In Martin Luther's letter to the Pope asking him to halt the practice of selling "get out of purgatory passes", there is the distinct implication that the Pope allowed it.
You are presuming that Luther "implied" anything of the sort. The practice was not universal, first of all, but relegated to one section of Germany. Next, Indulgences are not "get out of Purgatory passes", but rather, penance that relieves the punishment due to the temporal effects of sin. Some people could fast, others would pray, and others would give alms, to each their accord and position in life - given to them by God, no doubt. In any case, they are witness to the sorrow and contrition of the penitent. Were there abuses? Certainly. Was Luther correct in bringing the attention of this to the Pope? Sure. Was this a reason to leave the Church established by Christ? No.
Regards
I did like his phrase though in his letter to the Archbishop of Mainz: "the unhappy souls believe that if they have purchased letters of indulgence they are sure of their salvation; again, that so soon as they cast their contributions into the money-box, souls fly out of purgatory."