And a man who becomes unclean and does not purify himself, that soul shall be cut off from the midst of the congregation, because he has defiled the sanctuary of Hashem; the water of purification was not thrown upon him, he is unclean.
Torah Temimah: Makkos 8a
English: Rav Adda bar Ahava said to Rava: According to you, wherever it says “who” it means optional. Then regarding “a man who becomes unclean and does not purify,” only one who could choose would be liable, and one who contracted impurity through a meis mitzvah would be exempt? He answered: there it is different, for the verse says “he shall be unclean” — in any case.
Clarification of voluntary vs unavoidable tumah.
The verse still binds even where impurity was unavoidable; the phrasing does not free a meis-mitzvah case from the law of purification.