كَانُوا۟ لَا يَتَنَاهَوْنَ عَن مُّنكَرٍ فَعَلُوهُ ۚ لَبِئْسَ مَا كَانُوا۟ يَفْعَلُونَ
Verse seventy-eight speaks of a curse; verse seventy-nine gives the reason. And the reason given is not the committing of the evil — it is lā yatanāhawn, that is, their not restraining one another.
I record this as my own reading, and its ground is the form itself: the verse reproaches not an individual sin but a collective silence.
The same measure was treated at Hūd 11/116 (fa-law lā kāna mina'l-qurūni min qablikum ulū baqiyyatin yanhawna ʿani'l-fasād) and had occurred within this very sūra at al-Māʾida 5/63 (law lā yanhāhumu'r-rabbāniyyūn).
Three verses use the same verb for the same business — and this is a repetition that can be verified from the text.
تَرَىٰ كَثِيرًا مِّنْهُمْ يَتَوَلَّوْنَ ٱلَّذِينَ كَفَرُوا۟ (80) — and again kathīran minhum. And verse 81 gives the ground: wa law kānū yuʾminūna bi'llāhi wa'n-nabiyyi wa mā unzila ilayhi ma'ttakhadhūhum awliyāʾ — the bond of protection is measured by faith.