Why must an apostate make up missed prayers, unlike an original disbeliever, in the Shafi'i school of thought?

The Shafi'is hold that if an apostate repents and returns to Islam, it is obligatory for him to make up all the prayers he missed during his apostasy. They cite the hadith: "Whoever forgets a prayer, let him pray it when he remembers it..." as their evidence, because it applies to the apostate both in its wording and its concept. He is like one who deliberately abandons prayer. They differentiate between him and an original disbeliever by stating that the apostate is more intensely addressed and bound by the commands of the Sharia, and apostasy has nullified his deeds. Furthermore, he had previously committed to and acknowledged the commands of the Sharia, so his apostasy cannot be an excuse to evade making up what became obligatory upon him.