If the Jansenists did not deny the gratuity of grace, thinking grace cannot be earned or merited, why did they practice penances sometimes considered harsh?
Réginald Garrigou-Lagrange, O.P., Three Ages of the Interior Life, pt. 2 "Purification of the Soul in Beginners", ch. 19 "Practical Naturalism: Of Action and of Inaction" (ref:35.21):
In the seventeenth century the Jansenists fell into a pessimism which is an alteration of the Christian doctrine of penance. Like the first Protestants, they exaggerated the results of original sin to the point of saying that man no longer has free will, the liberty of indifference, but only spontaneity, and that all the acts of infidels are sins.759 They taught that “all his life long, a man must do penance for original sin.”760 As a result, they retained souls during a whole lifetime in the purgative way, and kept them away from Holy Communion, saying that we are not worthy of such a union with our Lord. According to their doctrine, only those should be admitted to Holy Communion who have a pure, unalloyed love of God.761 They forgot that this very pure love of God is precisely the effect of Communion, when it is accompanied by a generous struggle against all that is inordinate in us. Jansenism never attained to deliverance and peace.762
759. Cf. Denzinger, nos. 1094 [Cum occasione, May 31, 1658, condemned: "3. In order to merit or demerit in the state of fallen nature, freedom from necessity is not required in man, but freedom from external compulsion is sufficient."], 1291 [Condemned in a Decr. of the Holy Office, Dec. 7, 1690: "1. In the state of fallen nature, for mortal [Viva: formale] sin and for demerit that liberty is sufficient by which the mortal sin or demerit was voluntary and free in its cause, namely, in original sin and in the will of Adam sinning."], 1298 ["8. Of necessity, an infidel sins in every act."].
760. Ibid., no. 1309: “Homo debet agere tota vita poenitentiam pro peccato originali.” ["Man ought to do penance during his whole life for original sin."]
761. Ibid., no. 1313: “Arcendi sunt a sacra communione, quibus nondum illest amor Dei purissimus et omnis mixtionis expers.” ["23. Similarly, they must be prevented from Holy Communion, who have not yet a pure love of God, without any admixture."]
762. It has been said of Pascal that throughout his life he thought of sanctity without ever attaining it, because he remained in his own presence instead of in the presence of God.
It seems the Jansenists placed all the blame on Adam, thinking his sin was the only truly voluntary sin?